Spray three 8-inch square cake pans with vegetable cooking spray. Dust with flour. Prepare cake mix according to package directions, but substitute 1 cup of buttermilk for the water and increase eggs to 4.
Spoon half of the batter into a freezer-weight zip-top bag. Stir a few drops of pink icing color into remaining batter. Spoon pink batter into another bag. Snip a 1/2-inch corner from each bag. In two of the cake pans, pipe a 1-1/2-inch-wide strip of white batter around the outer edge of the pans, then a 1-1/2-inch-wide strip of pink batter; fill in center with white batter. For the third pan, pipe a 1-1/2-inch-wide strip of pink batter at outer edge, and follow with a 1-1/2-inch-wide strip of white batter; fill in center with pink batter. Bake as directed for 8-inch cakes. Transfer to wire rack; let cool 5 minutes. Invert onto wire rack and let cool completely.
To assemble cake, place one vanilla-centered cake layer on plate; spread with a thin layer of frosting. Top with the reverse-design cake layer; spread with another thin layer of frosting. Top with remaining cake layer. Frost entire cake.
Tint half of the remaining frosting pink, and the other half purple. Spoon frostings into freezer-weight zip-top bags fitted with #5 round pastry tips. Using tinted frostings, pipe grid design on cake as pictured. Pipe large dots of frosting, alternating colors, around bottom edge of cake. Place candy hearts on cake like checkers.