To make the coffee sponge, preheat the oven to 180 degrees C (350 degrees F). Line the bottom and sides of 2x8-inch springform/loose-bottomed round tins.
In the bowl of a stand mixer, beat the Egg (8) with both Brown Sugar (1/2 cup) and Vanilla Sugar (1/2 cup) till thick and mousse-like, 7-10 minutes.
Add the Coffee Extract (1 teaspoon), Instant Coffee (3 tablespoon), and Vanilla Extract (1 teaspoon) and beat in again for a minute, then beat in the Baking Powder (1 teaspoon) and Salt (1 pinch).
Sift the All-Purpose Flour (1 cup) in 3 batches, folding gently until it’s just mixed. Be careful not to release the beaten-in air.
Turn the batter into the prepared baking tin, and bake for 30-35 minutes, till a tester comes out clean.
Cool completely on racks, then slice into two horizontally, for a total of 4 layers.
To make the coffee syrup, place Corn Starch (2 tablespoon), Water (1/3 cup), Granulated Sugar (1/2 cup), and Instant Coffee (1 tablespoon) in a small pan and whisk until sugar dissolves and mixture thickens slightly. Cool.
To make the coffee mascarpone filling, whip the Cream (250 gram), Powdered Confectioners Sugar (50 gram), and Instant Coffee (3 teaspoon) together till medium stiff peaks form. Fold in the Mascarpone Cheese (250 gram). Taste and adjust sweetness if required.
To make the dark chocolate ganache, place chopped 52% Couverture Chocolate (450 gram) and Half and Half (300 gram) in a heatproof bowl and run at high in the microwave for 1 minute. Whisk until smooth, then leave it to rest to cool. Whisk again before using.
To assemble, brush each layer with the coffee syrup, and sandwich with the filling.
Give the whole cake a thin layer of ganache as a base coat. Let it set for about 5 minutes.
Frost with the remaining ganache. Sift over with Unsweetened Cocoa Powder (to taste) and garnish with Chocolate Shavings (to taste) and Coffee Beans (to taste).
Chill for about an hour or overnight. The flavors mature nicely over time. Slice and serve!