Divide each cake layer in two, horizontally, using a cake leveler or serrated knife. You should have six thin cake layers.
Place a dollop of frosting on the cake board or cake plate. Place the first cake layer, top-side up, on the cake board or cake stand.
Spread about 1/2 cup of buttercream across the cake layer, leaving about an inch clear around the edge of the cake.
Pipe buttercream dollops around the entire edge of the cake, using the Wilton 8B tip.
Fill the center with about a half cup of the pumpkin cheesecake filling.
Sprinkle 1/3 cup of the Biscoff crumble on top and pat into place.
Place the second cake layer on top of the filling and repeat these steps with the remaining 5 layers of cake.
Place the final cake layer, top side down, on the filling.
Decorate the top of the cake using the remaining buttercream and crumble.