Preheat oven to 350°F / 175°C. Line the bottom and the sides of three 8″ (20cm) baking pans with parchment paper. Set aside.
Make the genoise cake: In a large mixing bowl, using a handheld or stand mixer fitted with a whisk attachment, whisk eggs on medium speed just until combined. Add sugar and whisk on medium-high speed for about 10-12 minutes until white pale, foamy and tripled in size. (It takes 3-5 minutes longer with a handheld mixer compared to a stand mixer). To know if you have the right consistency, let a bit of batter drip off the whisk attachment on top of the mixed batter. The dripped batter should be visible for 10 seconds before it sinks into the batter. If it sinks earlier, you need to whisk longer.
With a wooden spoon, fold in flour, cornstarch baking powder carefully. Don’t overwork the batter and don’t work too fast otherwise, it will lose too much air. Divide in prepared baking pans and bake for 20-22 minutes until a toothpick centered in the middle comes out clean. Let cool in the pans for 5 minutes. Then remove from pans and remove the paper immediately. Transfer to a wire rack and let cool completely.
Make the mascarpone frosting: Whisk mascarpone on medium speed until creamy for about 2 minutes. Add powdered sugar and whisk until creamy and combined another 1-2 minutes. Add coffee and whisk until well combined and creamy for about 2-3 minutes. Stir in heavy cream and whisk until fully combined and creamy for another 2-3 minutes.
Assemble the cake: Place the first cake layer on a cake board or cake stand. Brush with a third of the 1/2 cup coffee.* Then spread 1/3 of the mascarpone cream with an offset spatula on top. Repeat one more time. Place last cake layer on top and lightly frost the outside and the sides of the cake with the remaining frosting. Level the top and sides with the offset spatula. Chill at least 4 hours in the fridge.
Pipe whipped cream on top if desired. Then dust with cocoa just before serving. Store leftovers in an airtight container in the fridge up to 3 days.