Separate the eggs, making sure no yolk gets into the whites. Beat the yolks with half the sugar until the mixture turns pale and lightly fluffy.
In a clean bowl, whip the egg whites with a pinch of salt. Slowly add the rest of the sugar and keep whipping until stiff, glossy peaks form.
Gently fold the whipped whites into the yolk mixture in two batches. Sift in the flour and cornstarch in stages, folding carefully so the batter stays light and airy.
Pour the batter into a piping bag or zip-top bag with the corner snipped. Pipe into a rectangle on parchment paper, dust with powdered sugar, and bake at 350°F (175°C) for 12 to 14 minutes until lightly golden.
Let the sponge cool fully. Trim it to fit your mold. Even a cut-open milk carton works great here.
In a separate bowl, whisk the egg yolks and powdered sugar by hand until smooth. Then mix with a hand mixer until pale and thick.
Add the mascarpone and beat until the cream is smooth and fully combined. Don’t overmix or it can go soupy on you.
Place one trimmed sponge layer in the bottom of your mold. Spoon brewed coffee over the top to soak it. Spread half the cream evenly over that layer.
Add the second sponge layer and repeat with more coffee. Finish it off with the rest of the cream.
Cover and chill for at least 4 hours, preferably overnight. Dust generously with cocoa powder before serving.