Make the cake base: Preheat oven to 350°F (175°C). Grease and line a springform pan. In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt. Add eggs, milk, oil, and vanilla. Mix until smooth. Stir in hot water or coffee. Pour into the pan and bake for 25–30 minutes. Let cool completely.

1. Prepare the mousse: Melt chocolate and set aside to cool slightly. In a small bowl, bloom gelatin in water (if using). In a saucepan, whisk egg yolks and sugar over low heat until thickened. Stir in gelatin (if using), then add melted chocolate and vanilla. Let cool slightly. In a separate bowl, whip heavy cream to soft peaks. Gently fold the whipped cream into the chocolate mixture until smooth.
2. Assemble the cake: Place the cooled cake layer back into the springform pan. Spread the mousse evenly over the cake. Smooth the top and refrigerate for at least 4 hours or overnight until set.
3. Finish and serve: Remove the cake from the pan and decorate with whipped cream, chocolate shavings, or fresh berries. Slice with a warm knife for clean cuts.