For the cake-
Preheat oven to 350°F. Prepare one of the cake mixes according to the package instructions. Add the food coloring of your choice and mix well. I used 5 different colors. You can decide how many colors you want, then divide your batter accordingly. Place the colored batter in gallon sized resealable plastic bags, cut a small corner and pipe the batter in rows onto a parchment lined large cookie sheet. The rows should be touching. Then bake for about 8-10 minutes. Do not overbake. Let cool completely. Use a heart shaped cookie cutter to cut your cake into cookies.
Prepare the batter from the other cake mix with no color. Pour a little in the bottom of a greased 9"x5" loaf pan. Line up your cookies, so that they are standing upright and right next to each other. They should be touching. Keep adding cookies until the pan is full of cookies from left to right. Pour the rest of the batter over the top of your colorful cookies. Bake for about 45 minutes or until golden brown. Let cool. Frost the cake with buttercream frosting. Add sprinkles for more color and have fun cutting into this beauty!
For the frosting-
Place shortening in large bowl and beat with an electric mixer on medium high for two minutes. Add the butter and beat until combined. With the mixer on low speed, gradually add half the sugar. Add the milk and vanilla. Beat on low for a few seconds. Keep mixer on low and add the remaining sugar. Cover mixer with a cloth, turn the mixer up to medium high and beat for 2 minutes. Add more milk if it's too thick.