Level each cake layer by using a serrated knife to make each one completely smooth on the top.
Place the first cake layer onto a cake stand or cake board using a small amount of frosting under the cake layer to keep it from sliding on the board.
Place half of the mousse over the first layer and spread it evenly over the cake and all the way to the sides.
Repeat with the next cake layer and the rest of the mousse and top it with the third cake layer.
Cover the cake with plastic wrap lightly and refrigerate the cake for at least an hour to let the mousse set up before frosting it.
Once chilled, do a crumb coat of the raspberry buttercream, lightly frosting the top and sides of the cake. Chill for at least 15 minutes.
After the cake has chilled for 15 minutes, frost the cake with the rest of the buttercream, smoothing the tops and sides as you prefer.
Use a spoon to pour a little bit of the slightly cooled ganache on the edge of the cake, letting it drip down the sides. Use the spoon to help it drip if needed. Repeat this for as many drips as you would like around the cake. If your drip is running too quickly down the cake and pooling at the bottom then let it cool a little longer before doing the rest of the drips. Fill in the top of the cake with more ganache, being careful to not let any more drip down the edges.
Decorate the cake with more frosting as desired. I piped swirls of frosting onto the top of the cake using a 1M open star tip and then whipped the remaining chocolate ganache with a hand mixer and piped those alternately with the buttercream.