Preheat the oven to 170 C (150 C fan oven, 325 F, gas 3). Grease and line 2 x 15cm (6 inch) round cake tins.
Cut 2 strips of parchment about 5cm (2inches) wide by 5cm (2inches) longer than the width of the tin – these will be used as tin dividers.
Sift the flour and baking powder into a bowl.
Put the butter and sugar in a second bowl and using an electric whisk, beat until creamy and well blended.
Gradually add the milk and sieved flour, in alternate batches, to the butter and sugar bowl, mixing until well blended.
Make up the egg white powder according to the packet instructions and whisk until softly foaming but not too stiff.
Gently fold the eggs whites into the cake mixture, taking care not to overmix the batter at this stage.
Divide the cake batter equally between 4 bowls.
Squeeze approx. ½ tube of a different Food Colour Gel into each portion and gently mix each to make vibrant coloured batters.
Take one coloured batter and spoon into half of the prepared tin, taking care to contain it within the area. Using one of the parchment strips, place it upright down the centre of the tin, and use to hold the batter in place, scooping it back if necessary, and tucking the excess parchment round the inside of the tin to make a division. Take another colour and spoon into the other side of the tin – keep the parchment divider in place during baking so that the mixtures cook as separate portions.
Make up the other tin in the same way with the other 2 portions of cake batter.
Bake for about 35 minutes until very lightly golden and just firm to the touch. Cool for 5 minutes then turn on to wire racks, remove the parchment and leave to cool completely.
Working on one cake colour at a time, use a 3cm (1 ¼ inch) round cutter, stamp out approx. 7 rounds from each piece of sponge.
Arrange one sponge circle upright in the centre of each tin, then lay another 4 pieces on their sides, close together, alternating in colour, around the centrepiece.
Lay the remaining pieces on their sides towards the edge of the tin – you should have room for 8 more pieces. Try and keep the pieces a little way in from the edge of the tin for best results during baking. Set aside.