Beat the 300g butter in a bowl until light and fluffy.
Sieve in the icing sugar and mix well being careful not to over beat the icing mix.
Stop when the buttercream is smooth and not at all grainy.
Carefully stir through a tsp of vanilla paste.
Spread a little buttercream onto each of your cake boards.
Place your bottom layer of each size cake on to the appropriate cake board.
Pipe a ring around the top of each cake. This will create a dam for the jam.
Spoon the jam on top of the cake up to the piped buttercream ring.
Take the second layer of each cake.
Spread the bottom with a little buttercream and place on top of the bottom layer.
Push down firmly (a little buttercream may push out the sides).
Using a spatula spread any excess buttercream around the sides of each cake.
Mix a little milk into the buttercream to make it more spreadable.
Using a spatula, spread a thin layer of butter cream over both cakes to catch any crumbs.
Place in the fridge for 30mins until the layer of buttercream has formed a crust.
Again using a spatula, spread on a thicker layer of buttercream on each cake. Try to keep the edges as flat as possible and create a sharp edge between the sides and top of the cake.
Place in the fridge after each layer has firmed up and repeat until you have used the buttercream.
Keep the cake in the fridge overnight (or for at least 2 hours) before attempting to cover in fondant.