Make the sponges: In a large bowl, beat together all the sponge ingredients then divide equally between the greased cake tins. Bake for 50-60 minutes or until cooked through (if it gets a little dark cover with foil). Remove from the oven and allow to cool for 20 minutes before turning out onto a wire rack to cool completely.
Make the buttercream: Beat the butter, vanilla and icing sugar together until light and fluffy. Add a drop of colouring to make a grey buttercream.
To assemble: Cut each cooled sponge in half horizontally, so you have 4 pieces of cake. Starting with one of the rounded pieces, use a bit of buttercream to stick it to your cake stand, rounded side down. With the cut side facing upwards, spread a layer of buttercream on to the sponge. Take the next piece of sponge (the piece that you cut the base from) and using a small cookie cutter, cut a hole right in the centre of the sponge, then stick it on top of the base sponge. Repeat this process with the other centre bit of cake, sticking it down with another layer of buttercream. The two holes that you’ve cut should match up and you should have a cavity running down the centre of the cake. Fill this with the sprinkles, then stick the last piece of sponge on the top with a layer of buttercream. You should be left with 4 layers of sponge that’s a perfect sphere in shape.
To decorate: Cover the sponge with the remaining buttercream and working quickly, arrange the squares of white chocolate in a circular pattern to cover the entire cake. Spray the cake with edible silver colouring, then dust with edible silver glitter.