Place your cake board on the turn table and place one of the cake layers (with a hole cut out) in the center of the cake board - you can use a small dab of buttercream to stick the cake layer to the cake board.
Pipe a layer of buttercream over the cake, then place the next layer of cake on the buttercream. Pipe another layer of buttercream and place the 3rd layer of cake on top.
Check that your cake is straight, then pop it in the fridge for 20 minutes or (freezer for 5 minutes) to firm up slightly.
Take the cake out of the fridge and fill the cavity with Cadbury mini eggs. Pipe a final layer of buttercream on top of the cake and pop the top cake layer on top. Then pop the cake in the fridge or freezer again to chill.
Take the cake out of the fridge and cover it with buttercream. To get smooth edges and sharp sides, first use your offset spatula to spread a generous layer of buttercream over the top of the cake, then use your cake scraper to smooth the edges while you spin the cake on the turn table.
Let the cake chill in the fridge for 20 minutes or (freezer for 5 minutes), then apply a second coat of buttercream. Place the cake back into the fridge or freezer to set again.