Heat oven to 350°F. Grease and flour two 8 or 9 inch round baking pans. (I like to line the bottom of mine with wax or parchment paper).
Stir together sugar, flour, cocoa powder, baking powder, baking soda and salt in large bowl.
Add eggs, milk, oil and vanilla; beat on medium speed of mixer 2 minutes.
Stir in boiling water (the batter will be thin). Pour batter into prepared pans.
Bake for about 30 to 35 minutes or until a toothpick inserted in the center comes out clean.
Allow cake to cool in the pan for 5 minutes and then remove to wire cooling racks. Cool completely. (You could freeze the cake rounds at this point, if desired. See note above)
For the chocolate mousse filling:
Stir the cocoa powder and hot water together. In a double- boiler (or in the microwave) melt the chocolate chips, just until smooth. Add the cocoa mixture to the melted chocolate chips and stir well to combine. Allow to cool to room temperature.
In a separate large bowl, beat the cream and sugar until very stiff peaks form. Add the chocolate mixture and fold in with a spatula until well combined. Refrigerate until ready to use, up to 5 days.
For the chocolate frosting:
In a medium saucepan, add milk, cocoa, and butter. Bring to a boil. Remove from heat.
Add powdered sugar and mix with electric beaters to get rid of any lumps. Allow to cool for 15-20 minutes, stirring occasionally.
Once cakes have cooled, use a sharp serrated knife to torte each cake (cut each cake evenly in half, horizontally, so that you end up with four, thin, 9'' cake pieces).
Place your first cake layer on your cake serving plate. Spread a big spoonful of mousse on top of that cake layer. Repeat with the remaining cake layers, adding a layer of mousse filling between each layer of cake.
Use remaining mousse filling to spread it evenly around the entire outside and top of the cake. Refrigerate for 30 minutes.
(I usually make the warm chocolate frosting while the cake is in the fridge.)
Remove cake from fridge and very slowly pour the cooled chocolate frosting over the top. Use a spatula or knife to gently spread the frosting evenly over the outside of the cake as you pour.