Prepare a 9 inch by 9 inch square tin by lining it with parchment paper or thick tin foil.

Crush the shortbread biscuits into crumbs - you can do this using a food processor, or by putting the biscuits into seal-able bag and bashing them with a rolling pin. Melt the butter and stir into the shortbread crumbs, then pour everything into the tin. Use the back of a spoon or your hand to push it down firmly and evenly into the tin. Pop into the fridge to set.

Put the butter and sugar into a large saucepan on a medium heat, until they're melted together. Add the can of condensed milk and put it on the highest heat, bringing everything to a rolling boil. Keep on a high heat for 2-3 minutes while stirring quickly, then remove from the heat and set aside for a couple of minutes.

Pour the caramel over the base, then pop back into the fridge while you melt the chocolate.

You'll need a few small bowls for this - I used mugs, or you can use small tupperware containers too! The first job is to melt 200g of your white chocolate in a heatproof bowl in the microwave (on 20 second intervals before stirring as white chocolate burns FAST). Once it's melted, spread over your caramel layer and set aside.

Begin making your colourful white chocolate! To make your first colour, put a handful (around 40-50g) of white chocolate chips/chunks into a mug or bowl and melt very carefully in the microwave. Once melted, add a few drops of vegetable oil until it's slick and liquidy, then begin adding gel food colouring until you have a colour you're happy with. Add spoonfuls of that colour to the top of your caramel shortbread.

Repeat the process with as many other colours as you want - 4 or 5 is manageable. Once all the blobs (see the pictures!) are done, use the end of a knife to swirl them together.