Coffee Chiffon Cake:
Separate 4 eggs into yolks and whites, and let them come to room temperature (about 30 min). Then preheat the oven to 350 degrees Fahrenheit.

Line a jelly roll pan (10.5" x 15.5") with parchment paper. Do not grease the pan or the parchment paper.

Prepare half a cup of coffee however you would for drinking, and let it cool to room temperature. If using instant coffee products, I'd suggest mixing half a cup of boiling water with 2 tsp of instant coffee, or 1 teaspoon of instant espresso powder.

In a large bowl, sift together 1 cup of cake flour, 2 tsp of baking powder, 1/3 cup of sugar, and half a teaspoon of salt.

Add in 4 egg yolks, 1/3 cup of vegetable oil, 1/2 cup of the prepared room temperature coffee, and 1 teaspoon of vanilla extract. Beat this together with an electric mixer just until smooth and well blended.

In a separate bowl glass or stainless-steel mixing bowl, add in the 4 room temperature egg whites and 1/2 tsp cream of tartar, and beat on low speed until it's frothy. Then increase to medium speed, and very slowly add in 1/3 cups sugar while continuing to beat. When all the sugar has been added, increase to high speed, and continue beating until you see stiff glossy peaks.

One-third at a time, very gently fold in this meringue mixture into the coffee batter cake mixture until it is well combined. Pour this batter into the prepared jelly roll pan, and smooth the top with the spatula.

Bake for about 15 to 18 minutes or until the surface springs back when gently pressed.

Dust the cake with some powdered sugar to prevent sticking. Place a rectangular piece of parchment paper that's bigger than the cake on top, and invert the cake onto the parchment paper and slowly remove the baking pan. Then carefully peel off the parchment paper the cake was baked with.

While the cake is still warm, dust additional powdered sugar onto this side of the cake (to prevent cake from sticking), and roll the cake with the parchment paper along the short 10.5-inch side, place with the seam side down so it doesn't unroll, and let it cool to room temperature on a wire rack.

Mocha Swiss Buttercream:
Take 2 cups of unsalted butter, cut it into 1-inch cubes, and let it come to room temperature.

In a large heat-proof bowl like glass or stainless steel, add in 5 egg whites, 1 1/2 cups of sugar, and 1 tsp of salt. Whisk together until they are combined.

Set up a double boiler and heat the egg white mixture in a low heat water bath while continuing to whisk, till the sugar is dissolved (when it feels sticky to touch, and you hardly feel the grains of sugar anymore). If you have a thermometer, it needs to reach 160 degrees Fahrenheit.

Take it off the water bath. Using an electric mixer, beat this mixture until glossy and stiff peaks form. Let it cool down completely to room temperature.

When it is back to room temperature, continue beating and slowly add the 2 cups pre-cut room temperature unsalted butter one cube at a time. The mixture might start looking curdled, but do not worry, and continue mixing with the electric mixer until it looks smooth.

When the butter is fully mixed in, add 1 tbsp of vanilla extract and 5 tsp of instant espresso powder. If instant espresso powder is not available, substitute by dissolving 3 tablespoons of instant coffee into 3 tablespoons of hot milk, letting it cool to room temperature, and then mixing it into the buttercream frosting (most instant coffee granules will not readily dissolve into frosting).

Mix this in until it's smooth. Scrape down the bowl to ensure that everything is mixed in. Leave this in the fridge until you're ready to use it.

Caramel Drizzle:
In a medium saucepan over medium-high heat, add 1/2 cup sugar and 4 tsp water. Do not stir the sugar. Just gently swirl the pan to make sure sugar is heating evenly.

Bring the sugar-water mixture to a boil until it turns into a copper/amber color. Be careful not let it get too dark, otherwise it will have a slightly bitter burnt flavor. Turn off the heat as soon as it turns light amber. It will continue to darken with residual heat.

Then while vigorously whisking, very slowly add in 1/3 cup of room temperature heavy cream, and vigorously whisk it in. Be careful of the hot steam from the initial addition of heavy cream. Adding the heavy cream very slowly will prevent the caramel from seizing up. But if the sugar does seize up into a solid ball, don't worry. Just keep whisking and it will melt again.

Add 2 tablespoons of butter and 1/4 teaspoon of salt, and turn on the heat to medium-low. Mix in the butter and continue whisking for 3 minutes.

Let it cool to lukewarm.

Assembling the cake:
When the cake has cooled to room temperature and while the caramel drizzle is cooling, slowly and carefully unroll the cake.

Spread a thin layer of the Mocha Swiss buttercream frosting on the top surface of the cake, about 1/4 inches thick.

Slowly and carefully roll the cake up again, the same direction it was rolled before but this time without the parchment paper rolled into it.

Transfer the roll to your serving dish, with the seam-side down, so it doesn't unroll. Fill the gaps at the ends of the cake with the Mocha frosting. Then cover the outer surface of the cake with a thin layer of frosting, as a crumb coat. Smooth it out with a bench scraper, and let your cake rest in the fridge for 20 minutes for the frosting to harden a bit.

When your crumb coat has hardened a bit, apply a 2nd layer of frosting, about 1/4" thick on the outer surfaces. Smoothen it with a bench scraper. If you have an icing comb, you can create a pattern on the outside surface of the frosting at this point. Remember to save some frosting to make rosettes at the top of the cake in the end.

When the caramel has cooled to lukewarm, spoon 1 big scoop of caramel into a disposable piping bag or ziplock bag. Cut at a very small hole in one of the corners. Do some practice drizzles on a plate.

When you're confident about your drizzles, drizzle the caramel over your cake roll in a zig-zag pattern. Try to drizzle the caramel with a very quick and confident motion while squeezing the bag of caramel pretty hard to get a straight zig-zag pattern, and avoid getting a squiggly drizzle. Save the rest of the caramel to serve on the side with leftovers, ice cream, or other desserts. (It will stay good for over two weeks in an airtight container in the fridge).

Put the remaining mocha frosting in a piping bag with a large star tip in the end. Pipe out rosettes on top of the roll, leaving a very little 1/4-inch gap in between each rosette. I aim for 6 rosettes.

Finally, place chocolate batons behind each rosette - so in our case 6 chocolate logs, depending on how many rosettes you made.

Let the cake rest at room temperature for 1 hour for the cake to set. Or refrigerate for overnight. If refrigerating, let the cake come to room temperature (30 minutes to 1 hour at room temperature, depending on ambient temperature) before serving. This allows frosting and caramel to soften again, and for the cake to have a more vibrant mocha flavor. Enjoy!