Slice each of the cakes into three 1-in/3cm layers. Place one of the 1-in/3-cm layers on a cardboard cake round of the same size. Apply an even coating of mousse, using 6 oz/170 g. Top with a second layer, apply the same amount of mousse, and top with the third layer. Coat the top and sides of the cake with buttercream. Refrigerate until the filling has set, at least 2 h