Preheat oven to 180C.
Cream the butter and sugar until light and fluffy.
Beat in the eggs until well combined (don't worry if it looks a bit odd, it'll be fine in a moment).
Gently stir in the flour and vanilla. If you're using a mixer, do this slowly or by hand.
Split the mixture into 4 bowls (about 500g in each).
Leave one plain and colour the others with a tiny tiny bit each of Egg Yellow, Ruby and Grape Violet until they are each a pastel shade.
Prepare 4 (or 2 at a time if you only have 2) 7" tins. Oil them and line the bottom with baking parchment.
Spoon each of the mixtures into a tin and bake for about 25 minutes or until just cooked and a skewer comes out clean.
If you've only baked 2 of the sponges, wash them out, prepare them again and bake the remaining 2 lots of batter.
When the cakes come out of the oven, leave to cool in the tin for 5 minutes and then turn out onto a wire rack.
Meanwhile, make the icing. Beat the butter until very light, pale and fluffy (about 5 minutes in a mixer or 10 by hand).
Beat in the icing sugar until total combined and fluffy. Stir in the vanilla.
If the icing is a little stiff, add a few drops at a time of boiling water and beat in until fluffy and 'floppy'.
Now to colour the icing, spoon out 2 lots, each of 200g of icing into separate bowls. Colour these Egg Yellow, Ruby (pink) and Grape Violet (lilac) using a tip, end of a knife tip, of gel colour.
Now watch my instruction video! (Below.)
Level the cakes and then use the remaining, uncoloured icing to sandwich the cakes. Cut out the centre core. Fill with Mini Eggs.
Use the coloured icing to cover the cake with an ombre, graduated colour effect.
Dollop any remaining icing on top to 'glue' a small mountain of Mini Eggs on.
Enjoy!