For the cake:
1. Line a 9x9 pan with aluminium foil and spray the foil with non-stick cooking spray. A 9x9 pan produces a fudge that is a little over an inch tall. If you want taller fudge, using an 8x8 pan will produce pieces that are almost 2 inches high.
2. In a bowl, combine 6 ounces (about 1 cup) of chopped white chocolate or white chocolate chips, 1/2 cup sweetened condensed milk, a pinch of salt, and 1/2 teaspoon of vanilla extract. Microwave the bowl for 30 seconds, then stir. Continue to microwave the white chocolate in 15-second intervals until it is completely melted and smooth when you stir. This can take anywhere from 45-90 seconds, depending on the strength of your microwave. Watch it carefully, as white chocolate can easily scorch.
3. Once melted and smooth, add a few drops of purple gel food colouring and stir until the colour is dispersed.
4. Scrape the fudge into the prepared pan and smooth it into an even layer. It will be very thin, but remember we are making six layers in all!
5. Chill the pan to set the layer, for about 30 minutes in the refrigerator or 10 minutes in the freezer.
6. Repeat the process from step 2 again. This time, once your white chocolate is melted, add blue food colouring and stir until combined.
7. Pour the blue fudge over the purple layer and spread it out, then chill the fudge again.
8. Continue to repeat this process, adding green, yellow, orange, and red layers of fudge.
9. On your final layer, use Cake Angels Rainbow Sprinkles to cover the top of the fudge. Refrigerate the fudge for several hours until it is completely set.
10. When you are ready to cut it, remove the fudge from the pan using the foil as handles.
11. Use a large sharp chef's knife to cut the fudge into small 1-inch pieces to serve.