Preheat oven to 350°F (175°C). Grease and line the bottom of an 8-inch round cake pan with parchment paper.
In a large bowl, whisk together flour, cocoa, baking powder, baking soda, salt, and sugar. Add buttermilk, oil, eggs, and vanilla. Mix until smooth. Slowly pour in the hot water or coffee and whisk until fully combined (the batter will be thin).
Pour into the prepared pan and bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool completely and level the top if necessary.
For the mousse, heat 1 cup (240 ml) of heavy cream in a saucepan until just steaming. Pour over chocolate chips in a bowl, let sit for 2 minutes, then stir until smooth. Cool to room temperature. Whip the remaining 1 cup (240 ml) of heavy cream with powdered sugar and vanilla until stiff peaks form. Gently fold the whipped cream into the cooled chocolate mixture until fully combined and airy.
To assemble, place the cooled chocolate cake on a serving plate or inside a springform pan. Spread the mousse evenly over the top, smooth the surface, and refrigerate for at least 2 hours.
For the ganache, heat heavy cream until steaming, then pour over chocolate chips. Let sit for 2 minutes and stir until glossy. Add butter if desired. Let cool slightly and pour over the chilled mousse layer, letting it drip down the sides. Refrigerate for at least 1 hour to set.
Decorate with chocolate curls, berries, or whipped cream. Slice with a warm knife for clean cuts and serve chilled.
Serving and Storage Tips
