What I love about this recipe, you don't need to beat anything separately. Just use the all-in-one method, pour all of the ingredients into a large mixing bowl and whisk with an electric hand whisk until combined. Once combined, pour into the cake tins and make it as even as possible.
Bake in the oven for 20-25 mins until springy to the touch. Turn the cakes out onto a wire rack and decide which is going to be the base and which is going to be the topper. Turn the base upside down and leave to cool.
Meanwhile prepare the buttercream. Whisk the butter and vanilla extract and gradually add in the icing sugar until combined. If the mixture is too thick, add a little water, if it’s too thin, add some more icing sugar until it reaches a creamy texture.
Turn the top cake upside down and mark out a circle on the sponge using a sharp knife. Make sure you don’t cut all the way through the sponge, you want to go half way down and scoop out the insides. Do the same to the base cake.
Once you’re happy with both sponges, cover the outside edges of each layer with a light buttercream.
Fill with all the candy you want.
Make a little mound of sweets and when you’re happy with the amount put the topper cake on top like a lid. Press the edges down firmly so they sandwich together.
Coat the cake in buttercream and cover away. You can use multiple colours. I was too impatient so I just went with my trusted yellow and topped it with sprinkles.
Get your camera ready, and record the person's expression as he or she slices into that cake and hears the candy fall out.