Place the first chocolate cake layer, top side up, on a cake board or a cake stand.
Pipe a rim of buttercream on the top edge of the cake.
Evenly spread about 1/4-1-3 cup chocolate ganache over the cake layer, using an offset icing spatula.
Using a piping bag, pipe about a cup of the vanilla buttercream over the chocolate ganache in an even layer. Spread it out gently.
Poor about 1/4 cup of the caramel over the buttercream. (Make sure the caramel is not higher than the rim of buttercream or it will spill out).
Sprinkle about a 1/2 cup of the shortbread crumbs onto the caramel. Gently pat them into place with your hand.
Repeat these steps for the second layer of cake.
Place the third cake layer, top side down, on the second layer of fillings.
Apply a thin layer of the chocolate ganache around the entire cake. This is the crumb coat. Freeze the cake for 15-20 minutes to set the crumb coat. This helps stabilize the cake, as well as locks in the crumbs to make it easier to apply the remaining ganache.
Frost the cake with the remaining chocolate ganache. Top the cake with remaining caramel and shortbread crumbs.