Preparing the Chocolate Cake Base :

1. Preheat your oven to 160°C (320°F). Grease a 22 cm (9-inch) round springform pan and line the bottom with parchment paper to ensure easy release.

2. Melt the chocolate and butter together using a double boiler (bain-marie) or in the microwave in short intervals. Stir until smooth and set aside to cool slightly.

3. In a mixing bowl, whisk the eggs and sugar until the mixture becomes pale and slightly thickened, this step adds lightness to the base.

4. Incorporate the melted chocolate mixture (still warm, not hot) into the egg mixture, whisking continuously until smooth and well combined.

5. Add the coffee, then sift in the cornstarch and gently fold it in to avoid any lumps. This will help create a smooth, gluten-free batter.

6. Pour the batter into the prepared pan and bake for 15 to 20 minutes. The cake should be set but still slightly soft in the center, maintaining a moist texture.

7. Let the cake cool completely in the pan before adding the mousse layer. This is essential to prevent the mousse from melting or separating.

Preparing the Chocolate Mousse :

1. Whip the cold heavy cream into soft peaks using an electric mixer. Keep the whipped cream chilled in the refrigerator while preparing the rest of the mousse.

2. Melt the dark chocolate, milk chocolate, and butter together over a gentle bain-marie or in the microwave. Stir until completely smooth and let cool slightly.

3. In a separate bowl, beat the eggs with the powdered sugar for about 2 minutes until the mixture becomes pale and slightly frothy.

4. Add the cooled chocolate mixture to the egg-sugar blend, whisking just enough to create a smooth, unified base.

5. Gently fold in the whipped cream in several additions using a spatula. Use upward folding motions to preserve the light, airy texture of the mousse.

6. Pour the mousse over the cooled cake base still in the pan. Smooth the surface with a spatula or offset knife for an even finish.

7. Refrigerate for at least 5 hours, or until the mousse is fully set. This resting time is crucial for achieving the perfect texture and clean slices.