Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line three 8" cake tins (that are at least 2" deep)
Mix together the butter and the caster sugar until smooth
Add the eggs, two at a time, and mix between each addition
Then add the vanilla extract, and whisk in the self raising flour
Fold in the sprinkles
Divide the mixture equally between the tins
Bake the cakes for 35 minutes or until a skewer inserted in the middle comes out clean. Remove from the tins and leave them to fully cool on cooling racks
To make the vanilla buttercream mix the butter and icing sugar together. Once it starts to come together add the vanilla extract and milk, and mix until smooth. I use my electric hand whisk to do this but you can also do it by hand with a spoon. If the buttercream is too stiff, add more milk
If the cakes have domed on top, level them off with a cake leveller or a serrated knife
Stack the cake layes, piping or spreading buttercream between each layer
On the final sponge, smooth icing over the top and smooth it out using a palette knife
Then use a piping bag with a star nozzle to pipe swirls around the edge and fill the middle with more sprinkles!
Leftovers will keep in an airtight container in a cool place for 2-3 days