Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugar on med-high until pale and fluffy (approx 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition. Add cotton candy flavouring and mix well.
Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Separate batter equally into four bowls (I use a kitchen scale). Color 3 of the bowls of batter with fucshia, sky blue, and violet gels respectively. Spoon batter randomly into each of the 3 pans, swirl once or twice with a bamboo skewer to marble.
Bake for approx. 35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack.
Cotton Candy Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**
Add cotton candy flavouring and whip until smooth.
Place one layer of cake on a cake stand or serving plate. Top with approximately 2/3 cup of buttercream. Repeat with remaining layers and crumb coat the cake. Chill for 20mins.
Frost the top and sides of the cake and smooth with a bench scraper.
Divide the remaining frosting into 4. Leave one batch white and color the remaining three fuschia, sky blue, and violet.
Doing one color at a time -- scrape a bit of colored frosting onto the back of a small offset spatula and spackle this onto the cake in a random pattern. Repeat with the other colors until they are evenly, but roughly, dispersed.
Run your bench scraper along the sides of the cake again to smooth out the newly added colored frosting. Repeat as needed, adding more color or patching with white.
Do a pearl border along the top and bottom using a small round piping tip with the remainder of the white buttercream.
Add cotton candy to the top if desired.***
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. *** Cotton candy does not hold up well for long periods of time and will not hold up at all in the refrigerator. I recommend adding this just before serving.