Boston Cream Pie
Yellow Cake with Vanilla Custard Filling & Chocolate Ganache
Vanilla Custard Filling:
6 egg yolks [120 g]
1/2 cup granulated sugar [100 g]
1/4 cup cornstarch [30 g]
1/2 tsp table salt [2.8 g]
2 cups whole milk [480 mL]
2 tbsp unsalted butter [28.4 g]
1 tbsp vanilla extract [15 mL]
1 tbsp dark rum (optional) [15 mL]
Yellow Vanilla Cake:
1 3/4 cups all-purpose flour [210 g]
2 tsp baking powder [8 g]
1/2 tsp kosher salt [3 g]
3 large eggs (room temperature) [150 g]
1 cup granulated sugar [200 g]
3/4 cup whole milk [180 mL]
8 tbsp unsalted butter [113.6 g]
1 tbsp vanilla extract [15 mL]
1/4 cup heavy cream [60 mL]
4 oz (2/3 cup) semisweet or bittersweet chocolate [113.2 g]
1/8 tsp kosher salt [0.8 g]
Vanilla Custard Filling:
In the bowl of your stand mixer, add in 6 egg yolks and 2/3 cup granulated sugar. Using the whisk attachment, mix them together on high speed until it looks thick and lemon-yellow in color.
Add in 1/4 cup of cornstarch and 1 tsp table salt, and continue to mix until fully combined.
In a medium saucepan, heat 2 cups of milk over medium heat. Once the milk is hot and steaming, turn off the heat, and while whisking on medium speed with the whisk attachment, add about one-third of the hot milk into the egg mixture, and mix them on medium speed until combined. Then while vigorously whisking by hand, add this milk-egg mixture back into the steaming milk in the saucepan, and whisk them together until well combined. It's important to constantly whisk so the egg doesn't not get cooked and form lumps.
Turn the heat back on and continue cooking over medium heat, and bring it to a boil, whisking constantly, until the pastry
cream is thickened. Once it starts to boil, continue heating for about 1 to 2 minutes. Transfer this mixture to a clean bowl and set it aside.
Whisk 2 tbsp of unsalted butter into the pastry cream, 1 tbsp of vanilla extract, and an *optional* 1 tbsp of dark rum until it is fully combined. If you prefer not to add alcohol, just leave out the dark rum.
Transfer the pastry cream into a large mixing bowl. By now the pastry cream should have cooled down to warm and no longer extremely hot. Cover it with plastic wrap so that the plastic wrap is touching the surface of the pastry cream to prevent a skin from forming. Rest this pastry cream in the fridge for a minimum of 3 hours to overnight.
Yellow Vanilla Cake:
Preheat your oven to 350 F (175 C). Take a round 9" by 2" cake pan, grease the sides and bottom of the pan with oil, butter, or nonstick spray, and line it with parchment paper. Grease the parchment paper as well. If you have a cake strip, soak it in cold water for 5 minutes. Shake off excess water but do not wring, and place it around the cake pan.
With a large mixing bowl and a sieve, add 1 3/4 cups of all-purpose flour, 2 tsp baking powder, and 1/2 tsp of kosher salt. Sift these together, and set aside.
In the mixing bowl of your stand mixer, add in 3 room temperature eggs and 1 cup of granulated sugar. With the whisk attachment, beat on medium-high speed until it looks thick and lemon-yellow in color (about 5 minutes).
In a microwave-safe container, ideally with a spout (e.g. glass measuring cup), add in 3/4 cup whole milk and 8 tbsp of unsalted butter. Microwave this for 2 to 3 minutes, in 30 second increments and stir it in between, or until the butter is completely melted and the milk is steaming. Alternatively, if you don't have a microwave, add the milk and butter to a small saucepan and heat over medium heat till it is steaming.
With the mixer on low speed, slowly add the dry ingredients mixture to the egg-sugar mixture, and continue beating until just combined.
Right before adding the steaming milk mixture into the mixing bowl, add 1 tbsp of vanilla extract into the steaming milk mixture. With the mixer on low speed, slowly pour the steaming milk mixture into the mixing bowl, and continue to mix just until fully combined. Stop the mixer, and scrape down the whisk attachment and the sides of the bowl, as needed.
This is called the "hot-milk method", where instead of adding the fat at the beginning of the mixing process, you first cream eggs and sugar together, then add in dry ingredients, and finally a hot mixture of melted butter and steaming milk. What makes this cake fluffy is when you beat the eggs and sugar for 5 minutes on medium-high speed, you're incorporating air bubbles into the eggs, which the baking powder will expand while baking. The hot butter-milk mixture coats the flour when added at the end, resulting in soft and even crumbles.
Spread this cake batter into an even layer in the prepared 9" cake pan, smoothing out the top as much as possible with your spatula. Gently tap the pan on the counter to remove any air bubbles. Bake this in the preheated oven at 350 F (175 C) for 30 to 35 minutes, or until the top surface looks light golden brown, and the top surface springs back when gently pressed.
Place the pan on a metal rack and let it cool for 2 minutes. If needed, run a thin knife around the edge to separate the cake from the sides of the pan. Then carefully invert it onto the metal rack, lift off the pan, peel off the parchment paper the cake was baked with, and let it finish cooling completely to room temperature.
Assembling the Cake:
Cut the cake in half horizontally, to make 2 layers. Place the bottom half of the cake on a revolving cake stand with the cut side up.
This step is optional, but if you want perfectly straight edges with the custard flush with the edges of the cake, take a piece of acetate sheet that's 4 inches high, and cut it about 30 inches long. Place it around the bottom layer of your cake, doing your best to make sure the edges are perpendicular to the cake stand surface. Tape the acetate sheet in place - we use the adhesive that came with the acetate sheet.
Take the chilled vanilla custard out of the fridge, and quickly whisk it to make it fluffy again and remove any lumps that may have formed. Spread the custard onto the cut side of the bottom half of the cake, making sure to go all the way to the edges and against the acetate sheet, creating a smooth and even layer of custard. You can also do this without the acetate sheet, the edges will just not be as straight and flush with the cake.
Place the top half of the cake, cut side down, onto the custard layer, squeezing it into the gap of the acetate sheet. Tap it gently with the back of your hand to make sure that the layers fit snugly. Rest this in the fridge to chill overnight for the custard layer to firm up.
The next day when you're ready to serve, prepare the chocolate glaze. In a microwave safe bowl add in 1/4 cup of heavy cream. Microwave for 45 seconds, until the heavy cream is steaming. If you don't have a microwave, you can also heat the heavy cream in a saucepan on the stove until steaming. Add in 4 oz of semisweet or bittersweet chocolate (about 2/3 cup of chocolate chips) and 1/8 tsp of kosher salt into the hot heavy cream, and whisk until the chocolate mixture is smooth and uniform. Let this chocolate ganache cool for 5 minutes.
Take the cake out of the fridge and remove the acetate sheet, revealing the layers of cake with vanilla custard in between. Spread the chocolate glaze over the top of the cake, using an offset spatula to create a smooth and even layer, letting some chocolate drip down over the edge of the cake. Let the finished cake rest at room temperature for 5 to 10 minutes for the chocolate glaze to set. Then slice and serve! For clean-looking slices, dip your knife in hot water for a few seconds, wipe water off the knife, and cut the cake - repeating this for each cut.
You can serve this cake as soon as the chocolate ganache has set. Leftovers will keep in the fridge for 2 to 3 days. For leftovers, we'd recommend taking it out of the fridge and letting it rest at room temperature for 30 minutes before serving, to allow the chocolate ganache to soften up. Enjoy!
NOTE: to get the sides and top/bottom surfaces of your cake to be more browned and have a "crusty" texture, use 1 1/2 cups of granulated sugar (instead of the 1 cup in the recipe). We prefer our yellow sponge cake to be less sweet, hence we only use 1 cup of granulated sugar. But the tradeoff is the sides and outer surfaces don't look as browned and crusty.