Cream the sugar and butter together in a large bowl until light in colour and fluffy in texture - about 3 minutes on medium speed.
Crack in the four eggs and pour in the vanilla extract. Beat for another minute on medium until well combined.
Sieve in the flour, baking powder, baking soda and salt. Beat on a low speed for about one minute until just combined - don't over-mix!
Divide the mixture as evenly as you can between seven bowls.
Add a couple of drops of food colouring to each bowl and mix very gently until the colour has been evenly distributed - again, do not over-mix!
Use a bit of oil or butter to grease your mug and scoop all the batter of one colour into it, smoothing down the top with the back of a spoon. I steamed each layer in the microwave on full power for 1 minute and 40 seconds. Test your cake after 1 minute 20 seconds by sticking a toothpick in the centre - if it comes out clean of crumbs or batter then it's ready. Keep cooking the batter for an extra 5 seconds at a time until you're happy it's done. When cooked, tip your coloured cake out onto a wire wrack and leave until completely cool.
After you've found out how long you need to cook each layer for, continue steaming each layer in the same mug until you've gone through all the colours - don't forget you need to grease the mug each time! Leave them all to cool together on a wire wrack.
When cooled, slice each coloured mug cake into four even slices.
Stack each cake up in the following coloured combination: red, orange, yellow, green, blue, purple and pink, sandwiching each layer with whipped cream. Serve asap - these cakes fall over!