Tint 3 tablespoons of frosting with yellow food coloring, spoon it into a ziplock bag, and snip off a corner. Tint the remaining frosting purple. Trim the top of one of the cakes to make it level. Set the cake on a sheet of parchment paper to catch extra sugar sprinkles. Cover the top of the cake with a layer of purple frosting, then place the other cake on top. Create the look of a tufted pillow by using a serrated knife to make an X-shaped indentation across the cake, corner to corner (the lines will serve as a guide for the stitching). Cover the cake with the remaining purple frosting, then coat it with purple sparkling sugar. Transfer the cake, on the cardboard, to a serving platter. Return any excess sugar to its container.