Preheat your oven to 350°F (175°C). Prepare your cake pans by greasing them and lining the bottoms with parchment paper.
In a large mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Whisk until well combined.
In a separate bowl, beat the eggs, granulated sugar, buttermilk, oil, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Carefully stir in the boiling water or hot coffee. The batter will be thin, but that’s okay! This ensures a moist cake.
Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to a cooling rack to cool completely.
In a heatproof bowl, melt the semi-sweet chocolate chips over a double boiler or in the microwave. Allow to cool slightly.
In a mixing bowl, whip the heavy cream, granulated sugar, and cocoa powder until soft peaks form. Gently fold in the melted chocolate until fully combined.
Once the cakes are completely cool, place one layer on a serving plate. Spread half of the chocolate mousse over the top. Place the second layer on top and cover with the remaining mousse.
Chill the cake in the refrigerator for at least 2 hours to set the mousse. Serve chilled, and enjoy each decadent slice!
