Food colouring gels: red, pink, green, purple, orange, blue
130g plain flour, sieved with 1/2 tsp salt
4 egg whites
4 egg yolks
125g caster sugar
Yellow food colouring gel
300ml double cream, whipped cream
Cream together the icing sugar and butter with an electric whisk. Add in the egg whites and whip until fluffy, then beat in the plain flour. Divide this mixture between six small bowls.
Use food colouring gel to colour each of the mixtures a different colour. You want them to be very vibrant. Then put each one into a separate piping bag - without a nozzle.
Line a swiss roll tin with non-stick baking paper. Secure the paper down with a small blob of cake mixture underneath each corner.
Snip a small hole in the end of each piping bag and carefully pipe your rainbow dots across the paper.
When you’ve finished, put the tray into the freezer for 15 minutes while you prepare the swiss roll mixture.
Preheat the oven to 180C (160C) fan.
In a clean bowl, beat the egg whites until they form stiff peaks.
In another bowl, whisk together the egg yolks and caster sugar, until light and fluffy. Add a big dab of yellow food colouring gel, and whisk that through - you want the yellow colour to be very bright.
Gently add the sieved flour to the egg yolk mixture and fold it through. Add 1/3rd of the egg whites and gently fold them in - taking care not to knock out the air bubbles. Then fold through the rest of the egg whites.
Remove the rainbow dot tray from the freezer - the dots should be starting to firm up and have a skin on them.
Carefully spoon the swiss roll mixture over the dots, and spread it around with a palette knife.
Bake for 10-12 minutes, until it’s just starting to brown and is springy to touch, and then remove it from the oven.
Prepare another piece of baking paper just bigger than the swiss roll tin. After a few minutes, when the swiss roll is cool enough to handle, loosen the edges then put the new piece of baking paper over the top of it and carefully turn the whole thing over, so that the tray is on top. Remove the tray, then very carefully turn it over again so that the new piece of paper is on the top, and the dots are underneath.
Starting at a short end, carefully roll up the swiss roll, peeling back the original piece of baking paper as you go and revealing the rainbow dots. Leave the swiss roll rolled up like this to cool.
Once the swiss roll is fully cooled, un-roll it and fill it with a layer of raspberry jam and then whipped cream. Gently roll it back up again, trimming the end so that it will lay flat, underneath, and trimming the sides to show the roll off neatly.