Preheat the oven to 180°C/gas mark 4. Grease and line three 18cm cake tins.
Place the butter and sugar in the bowl of a freestanding mixer and beat until very light and fluffy. Add the eggs one at a time, beating well between each addition.
Sift the flour into the butter mixture and carefully fold it through using a plastic spatula or large metal spoon. Add the milk and vanilla paste and stir through, then very carefully fold through the sprinkles.
Divide this mixture evenly between the three cake tins and level the surface. Bake for 25-30 minutes or until risen and golden and a skewer inserted into the middle comes out clean. Leave to cool in the tins for 15-20 minutes then remove and leave to cool completely on a wire rack.
To make the confetti crumb, rub the butter into the flour until you have very coarse crumbs. Add the sugar and sprinkles and stir to combine. Spread the mixture on a baking tray lined with greaseproof paper. Bake in the oven for 10-15 minutes until golden. Remove from the oven and leave to cool.
To make the buttercream, put the butter in the bowl of a freestanding mixer and beat until soft. Add half the icing sugar and beat until fully incorporated. Add the remaining icing sugar and vanilla paste and beat again until smooth.
To construct the cake, level each of the layers so they're even. Use the base of a cake tin as a template and, using a sharp knife, cut the very outer edges of the cake to remove the darker outer layer and expose the lighter coloured cake.
Spread the first layer with a third of the buttercream and sprinkle over a third of the crumb mixture. Top with another layer of cake and repeat. Finish with the final layer of cake. Swirl the remaining buttercream on the top of the cake and top with the remainder of the crumb mixture. Use a palette knife to smooth any excess buttercream around the sides of the cake.