Preheat oven to 350°F (175°C). Grease and flour two 8 or 9 inch round baking pans.

In a large bowl, whisk together the sugar, flour, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, whisk together the eggs, buttermilk, oil, and vanilla extract.

Add the wet ingredients to the dry ingredients and whisk until just combined.

Stir in the boiling water until the batter is smooth.

Pour the batter into the prepared baking pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 5 minutes before removing them to a wire rack to cool completely.

In a medium bowl, whisk together the cocoa powder and hot water until smooth.

In a double boiler or microwave, melt the chocolate chips until smooth.

Remove the melted chocolate from the heat and stir in the cocoa mixture until combined.

Allow the chocolate mixture to cool to room temperature.

In a large bowl, beat the heavy cream and sugar until stiff peaks form.

Gently fold the chocolate mixture into the whipped cream until just combined.

Refrigerate the mousse until ready to use.

In a medium saucepan, whisk together the milk, cocoa powder, and butter.

Bring the mixture to a boil over medium heat.

Remove the saucepan from the heat and whisk in the powdered sugar until smooth.

Allow the frosting to cool slightly before using.

Once the cakes have cooled completely, use a sharp knife to cut each cake in half horizontally.

Place one cake layer on a serving plate and spread with a layer of mousse.

Repeat with the remaining cake layers, alternating between layers of cake and mousse.

Spread the remaining mousse over the top and sides of the cake.

Pour the chocolate frosting over the top of the cake and spread it evenly with a spatula.

Refrigerate the cake until the frosting is set.

Enjoy!