• Preheat your oven to 325 °F (165℃) and prepare a jelly roll pan by greasing it well and lining the bottom with a piece of parchment paper. It's also a good idea to grease the top of the parchment paper.
• Sift the flour, cornstarch, cocoa powder, and baking powder in a bowl and mix them together well.
• Add the egg yolks, sugar, vanilla and espresso powder to a separate bowl and mix them on high with a hand mixer until the mixture is light and creamy and the sugar grains have disappeared.
• To another clean and dry bowl, add the egg whites with the salt and whisk them until soft peaks form. Add the cream of tartar and mix very well until you get strong and sturdy hard peaks.
• Over the egg whites, add 1/2 of the egg yolk mixture and 1/2 of the flour mixture and fold them together gently. Repeat the process with the rest of the egg yolk mixture, folding them as gently as possible to avoid deflating the egg whites.
• Pour the batter into the prepared jelly roll pan and spread it out to the edges being careful to spread the batter as evenly as possible. Bake at 325℉ (165℃ ) for about 13-15 minutes or until a toothpick inserted into the middle of the cake comes out clean.
• As the cake bakes, dust cocoa powder generously over a clean kitchen towel creating a rectangle shape, slightly larger than your jelly roll pan.
• Immediately after taking the cake out of the oven, drag a butter knife between the pan and the edges of the cake to make sure all the edges come loose from the pan. Line a cooling rack with parchment paper and place it upside down on top of the hot cake before inverting. Invert it again so the cake is face down on top the kitchen towel over the powdered sugar. Peel off the parchment paper from the bottom of the cake.
• Roll the cake with the towel inside the roll as well, from the narrow end, while it's hot from the oven. Don't roll it too tightly or aggressively, but the cake should be firmly rolled with no spaces on the inside of the roll.
• Let the cake roll cool, rolled, turning it several times throughout the cooling process so that it keeps a round shape and doesn't flatten on one side. This should take between an hour and an hour and a half.

For the filling

• Whip the heavy cream in a clean bowl on high speed with the powdered sugar until the mixture reaches medium to hard peaks. Be sure to whip the cream right before you're about to fill the cake roll.

Assembly:

• Once the whipped cream has reached the right consistency and the cake has cooled completely, gently unroll the cake (you don't need to flatten it completely - one end will have a small curl). With an offset spatula gently spread the whipped cream out evenly to a thickness of about 1/3-1/2 inch, leaving the last 1/2 inch of the cake on the narrow end (the flat end) free of cream. Add a little bit of extra cream under the curled up end.
• Starting with the curled end, gently roll the cake back up without the towel. Wrap it tightly in plastic wrap and put it in the fridge for an hour or pop it in the freezer for 20 minutes to set.

Serving:

• After the cake roll has set, remove the plastic wrap, cut 1/4-inch slices from each raw end of the cake roll.
• Slice the cake roll and serve.