For the Cake:
Bake cake according to package directions in a 18" x 13" sheet pan. Cool completely in the refrigerator or freezer. Cut two hearts using the large cutter. Punch out the inside of the heart cake using the smaller cutter, so you end up with a cake that looks like an outline of a heart. Chill completely, or better yet, freeze - the harder the cake is, the easier it will be to handle and assemble. (And save the leftover cake for a different heart cake!)
While the cake is chilling, make the frosting:
For the Frosting:
With a hand mixer or electric mixer, beat 1/4 cup of coconut oil until light, fluffy, and smooth. Slowly add 8 oz powdered sugar, the 1 tsp vanilla extract, and 2 tbsp of your milk of choice. Slowly add another 8 oz of powdered sugar, and then another 2 tbsp of your milk of choice
Beat until completely smooth and lump-free.
Separate frosting into small bowls, and use the food coloring to make different colors. Put each colored frosting into its own pastry bag (or, use a resealable sandwich bag and cut the corner off!).
Before using, ensure your cake is completely cool, as coconut oil melts easily.
Place one heart cake on a serving plate. Using your colored vanilla frosting, squeeze dots of frosting on the cake, in whatever arrangement of colors you'd like. Carefully place the second heart cake on top of the first frosted cake, and repeat the frosting process.