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 powder, baking powder, baking soda, salt, and sugar.
Add buttermilk, oil, eggs, and vanilla extract. Mix until smooth.
Gradually pour in the hot water or coffee and whisk until the batter is fully combined (it will be thin).
Pour into the prepared pan and bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool completely, then level the top if necessary.
2. Make the Chocolate Mousse:
Heat 1 cup (240 ml) of heavy cream in a saucepan until just steaming (do not boil).
Pour over the chocolate chips in a bowl, let sit for 2 minutes, then stir until smooth.
Allow the mixture to cool to room temperature.
Whip the remaining 1 cup (240 ml) of heavy cream with powdered sugar and vanilla extract until stiff peaks form.
Gently fold the whipped cream into the cooled chocolate mixture until fully incorporated and airy.
3. Assemble the Cake:
Place the cooled cake layer on a serving plate or inside a springform pan.
Spread the chocolate mousse evenly over the top, smoothing the surface.
Refrigerate for at least 2 hours to allow the mousse to set.
4. Prepare the Ganache Topping:
Heat the heavy cream until steaming (do not boil).
Pour over the chocolate chips and let sit for 2 minutes, then stir until smooth and glossy.
Add the butter if desired for an extra shiny finish.
Let cool slightly, then pour over the chilled mousse layer, allowing it to drip beautifully down the sides.
Refrigerate for at least 1 hour to set completely.
5. Decorate and Serve:
Garnish with chocolate curls, fresh berries, or whipped cream.
Slice with a warm knife for clean edges and serve chilled.
