Using a cookie cutter roughly 9 cm in diameter, cut out three circular cake layers.
Make the buttercream by beating together half of the butter, all of the icing sugar and the vanilla paste until smooth. Add the extra butter until the icing is a smooth, spreadable consistency but holds its shape.
Spread a little buttercream on to a small cake board followed by the first cake layer.
Pipe a ring of buttercream around the top edge of the cake layer, this will stop any jam spilling out the edges.
Add the jam inside this ring of icing and then place the second layer on top.
Repeat this until the final layer of sponge is stacked.
Using a spatula, cover the rest of the cake in a layer of buttercream (reserving a little to pipe on top).
Carefully press sprinkles into the sides of the cake until completely covered.
Heat the dark chocolate gently in either a bain-marie or with 10 second intervals in the microwave.
Leave the chocolate to cool slightly then, while still runny, spoon on top of the cake. Gently spread the melted chocolate to the edge of the cake and let drips fall down the side.
Once the chocolate has set, pipe swirls of the remaining buttercream and top with more sprinkles.