Beat the sugar and butter with the vanilla essence until light and fluffy (about 5 minutes).
Add 1 egg, 1/3 of the flour and a splash of milk and beat until just combine. Repeat until all these ingredients are combined.
Divide the batter into 2 tsp portions in the holes of your cupcake tin, then colour the first 6 portions red using your paste colouring. Colour the remaining 6 portions purple. Repeat with the other colours in separate cupcake tins. You will need 6 portions of each of the 6 colours to make the rainbow cake layers, so you may need to bake the cake portions in 3 batches of 12 if you only have 1 cupcake tin.
Bake in the oven for 5-8 minutes or until cake is springy to the touch, remove and cool in the tray for a few minutes before cooling on a wire rack.
Make the buttercream by adding all the ingredients together and beating for 5 minutes with an electric mixer on a medium speed.
When the cake is cool, use a circle cutter the same width as the push-up to cut the coloured sponge to the same size as the push up.
Sandwich the different coloured sponges with a touch of jam and buttercream until you have all 6 colours stacked. Donít put too much buttercream in as it will smear the push up when you put them in.
When you have a stack of 6, assemble a push-up and with the stack on the work surface push the tube down over the stack until it is all in the tube. Push the cake down slightly so it pushes the buttercream to the edge of the cake.
Add some buttercream and rainbow sprinkles to the tops and pop the lid on.