1. To make the ice cream layer, whisk together the condensed milk, double cream and vanilla until thick. Pour in to a cling film-lined cake tin the same size as the ones you’re going to bake the cakes in.
2. Grease and line two 10-inch cake tins.
3. Cream together the butter and sugar until pale and fluffy, slowly add in the egg whites and yolk and continue to beat until creamy. Add the vanilla. It might look a little split at this stage but keep going.
4. Sift together all the dry ingredients. Starting and ending with the dry ingredients, alternate adding dry ingredients and buttermilk to the mixture.
5. Using an ice cream scoop, divide the batter in to 5 bowls and colour each with the gel food colouring. Pale pastels or bright and vibrant - it's up to you.
6. Once your colours are combined, use an ice cream scoop to divide each colour in to the tins in concentric circles, so one colour on top of the other. I went in this order: green, purple, yellow, pink and blue.
7. Bake in a preheated oven at 180°C for 25-35 minutes depending on your oven, or until a skewer poked into the middle comes out clean. Remove from the oven and cool completely before wrapping in cling film and freezing.
8. To make the buttercream, place the softened butter in a stand mixer and beat until pale. Slowly add the icing sugar and vanilla and continue to beat for around 5 minutes until really light.
9. If you need to trim the tops of your cakes, do that now.
10. To assemble the cake, take a cake board, take your first sponge and top it with the ice cream layer you froze earlier, removing all the cling film. Top with the second layer of sponge.
11. Ice the whole cake with buttercream, filling in any gaps as you go. Decorate the edges with sprinkles. Freeze. Remove from the freezer 20 mins before you want to serve.