Puff Pastry Dough:

Take 12 tbsp of unsalted butter and let it come to room temperature. Divide it equally into 3 separate bowls (4 tbsp butter each), and whisk the butter so you have 3 bowls of spreadable butter. Set this aside.

In the bowl of a stand mixer fitted with the paddle attachment, add in 1 1/2 cups all-purpose flour, 1/2 cup of cool water and 1/4 tsp kosher salt. Mix on low speed, until the mixture comes together and has a tacky consistency.

Stop the mixer, switch to the dough hook attachment, and knead on low speed till it becomes a smooth ball. Cover this ball of dough with plastic wrap and let it rest in the fridge for 1 hour, or till it reaches 50 to 60 F (10 to 16 C).

Remove the plastic wrap and roll the dough into a 12" x 12" square. Spread butter from 1 of the 3 bowls on two-thirds of the dough surface, leaving a 1-inch rim around the edge of the dough. Carefully peel dough off the surface and fold the unbuttered third of dough over the middle. Then press gently to push any air bubbles out on the sides. Then fold the opposite end over like a letter. Straighten the edges as needed. Again, press gently to push any air bubbles out on the sides, pat the dough to seal the edges. Straighten the edges as needed. Transfer the dough to a parchment paper-lined baking sheet, cover with plastic wrap, and let the dough rest in the fridge for 1 hour, or till the butter reaches 50 to 60 F (10 to 16 C).

When the butter has cooled to 50 to 60 F (10 to 16 C), roll the dough into another 12" x 12" square. Again, spread butter from the second of the 3 bowls on two-thirds of the dough surface, leaving a 1-inch rim around the edge of the dough. Using a bench scraper, flip the unbuttered third of the dough over the middle and fold the opposite end over like a letter. Straighten the edges as needed. Press the edges to seal and pat the dough, releasing any air bubbles at the sides. Transfer the dough to a parchment paper-lined baking sheet, cover with plastic wrap, and let the dough rest in the fridge for another 1 hour, or till the butter reaches 55 to 60 F (13 to 16 C).

When the butter has cooled to 50 to 60 F (10 to 16 C), roll the dough into another 12" x 12" square. Spread the last bowl of spreadable butter all over the dough, leaving a 1-inch border just on the top edge, to help seal the dough. Starting with the edge closest to you, roll the dough into a tight log, sealing the dough at the unbuttered edge. Make the ends even and flat, as needed. Wrap in plastic wrap, place it on the parchment paper-lined sheet pan, and refrigerate until very firm, at least 2 hours, preferably overnight.

Ube Custard Filling:

In a medium saucepan, add 3/4 cup granulated sugar and 1/4 cup + 1 tbsp of water, do not stir. Heat over high heat. Bring to a boil (do not stir) and cook till it reaches 215 F (102 C) then remove from the heat and let sit until ready to use.

In a small saucepan, heat 1 cup of the milk over medium heat until bubbles begin to form around the edges. Let the hot milk cool for 10 minutes. In a large bowl, whisk 3 tbsp of all-purpose flour with the remaining 1/2 cup of milk. While continuing to whisk, add the hot milk in a slow, steady stream. Then whisk the sugar syrup into the milk mixture in a steady stream. Return everything back to the saucepan the sugar syrup was in, and cook over low-medium heat, whisking constantly, until it's thick enough to coat the back of a spoon, about 5 to 10 minutes. Turn off the heat. The custard should be quite thin and runny.

While whisking constantly, add 6 egg yolks, one yolk at a time, then add 1/2 tsp of ube extract to the mixture and whisk until well combined. Then strain through a fine-mesh sieve into a bowl or measuring cup with a spout. Press plastic wrap onto the surface of the custard so a skin does not form on top. Set aside to cool completely to room temperature.

Assembling and Baking the Tarts:

Place an oven rack in the middle position and preheat the oven to 475 F (245 C). Butter a standard 12-cup muffin pan (top diameter of 2.75" for each well)

Take the dough log out of the fridge, remove the plastic, and roll the log back and forth, elongating it, till it's exactly 12" long. Then cut it into 12 equal pieces. Place these pieces of dough in the fridge so they stay cool.

Taking one piece of dough at a time, let it warm up to 50 to 60 F (10 to 16 C). When it reaches that temperature, flatten it with a rolling pin and roll it into a disc that has a diameter of 5". Place the 5" diameter disc into the well of the muffin pan, and press it into the well so that the dough extends about 1/8" above the top rim of the muffin pan. Put the muffin pan back in the fridge to keep the molded dough cold while rolling out the next circle. Repeat this with the remaining dough, taking 1 piece of dough out of the fridge at a time, letting it warm to 50 to 60 F (10 to 16 C) and putting the muffin tin back in the fridge between each piece of dough. It's crucial that the butter remains cold, not warming above 60 F (16 C) when it's being shaped. When all the 12 wells are filled with the molded dough, rest the muffin pan in the freezer for a minimum of 30 minutes.

When the molded dough has chilled in the freezer for 30 minutes and the custard has cooled to room temperature, quickly pour the custard into the 12 wells, distributing them evenly, which should go up about 2/3 of the way up. Work quickly as you don't want to give the custard any chance of soaking into the pastry dough. It's important to have the oven ready to go once you start pouring custard into the shells.

Place the pan on the middle rack in the 475 F (245 C) preheated oven, close the oven door, and immediately turn the oven temperature down to 450 F (230 C). And bake for a total of 20 to 30 minutes, rotating the pan at the 15-minute mark for even browning. The tarts are done when the shells are golden brown and crisp, and the tops of the custard are browned in spots.

Let them cool in the muffin pan for 5 minutes. Then transfer the tarts to a metal rack to cool further. Serve warm. Reheat at 350 F (175 C) for 5 to 10 minutes. Enjoy!