Grease and flour two 8” cake pans or line with parchment paper.
Boil 1 cup water.
In a large bowl, stir together sugar, flour, cocoa powder, baking powder, baking soda and salt.
To that add: eggs, milk, oil, and vanilla extract. Use your mixer to beat on medium speed for about 2 minutes.
Slowly stir in the boiling water.
Pour the batter evenly in the cake pans.
Bake 30-35 minutes, or until a toothpick inserted comes out clean.
Leave the cakes in the pans for about 5 minutes to cool before transferring them to the cooling rack to finish cooling.
Chocolate Mousse Filling
Whisk together cocoa powder and hot water.
In the microwave, melt the chocolate chips just until smooth. Add the cocoa mixture to the chocolate chips and stir well. Allow mixture to cool to room temperature.
In another bowl using your mixer, beat the heavy whipping cream and sugar until stiff peaks form. Mix in the chocolate mixture until well combined and smooth.
Refrigerate until cake has cooled completely.
Chocolate Frosting
In medium saucepan, add milk, cocoa powder, and butter. Bring to a boil then remove from heat.
Mix in powdered sugar and use a mixer to smooth the frosting. Cool 10-15 minutes.
Chocolate Cake Assembly
Use a sharp knife and cut each cake layer in half, making 4 layers.
Place the first cake layer down on a cake plate, then spread chocolate mousse until even. Add another cake layer and spread more chocolate mousse. Keep repeating, making sure you spread mousse on the top of the cake as well. Then, spread a thin layer of chocolate mousse around the sides of the cake.
Refrigerate 30 minutes.
While cake is cooling, make the frosting.
Take the cake out of the refrigerator and pour the warm frosting over the cake. Use a spatula to spread the frosting while you are pouring it.
Refrigerate until frosting has set.
You can use any leftover chocolate mousse to pipe rosettes on the top of the cake (optional). Then top with chocolate sprinkles if desired.