Bake the Funfetti Cake according to the box instructions and let cool.

In a stand mixer with a paddle attachment, add butter and beat on high for 4 minutes, scraping the bowl once light and fluffy.

Add powdered sugar, salt, milk, and vanilla to the mixer and mix on low speed for 30 seconds.

Increase the speed to high until frosting is smooth; set aside.

Crumble 1/3 of the funfetti cake with your hands and place it in the freezer to firm up.

Once crumbs are cool and hold their shape, fold them into your frosting.

Once I frosted the cupcakes, I added birthday cake crumbs on top as well!