Preheat the Oven – Set to 350°F (175°C). Grease and line a 9-inch cake pan with parchment paper.

Mix Dry Ingredients – In a bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

Cream Butter & Sugar – In a separate bowl, beat butter and sugar until light and fluffy. Add eggs and vanilla, mixing well.

Combine – Alternately add the dry ingredients and buttermilk, mixing until just combined. Stir in hot coffee.

Bake – Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let cool completely.

Melt Chocolate – In a heatproof bowl, melt the chocolate and butter over a double boiler or in the microwave. Stir until smooth and let cool slightly.

Heat the Milk Mixture – In a saucepan, heat milk and sugar until warm. In a separate bowl, whisk egg yolks. Slowly pour the warm milk into the yolks, whisking constantly. Return the mixture to the saucepan and cook over low heat until slightly thickened.

Bloom the Gelatin – In a small bowl, dissolve gelatin in water. Microwave for 10 seconds to liquefy, then stir it into the chocolate mixture along with vanilla extract.

Fold in Whipped Cream – Whip the heavy cream until soft peaks form. Gently fold it into the chocolate mixture until smooth and airy.

Layer the Mousse – Spread the chocolate mousse evenly over the cooled cake base.

Chill – Refrigerate for at least 4 hours, or until the mousse is fully set.

Decorate – Dust with cocoa powder, add whipped cream, chocolate shavings, or fresh berries. Slice and serve chilled.