Rainbow Heart Surprise-Inside Cake

Name: Rainbow Heart Surprise-Inside Cake
Prep time: 25 min
Cook time: 55 min
Total time: 1 h 20 min
Cuisine: American
Ingredients: 2 boxes white cake mix (prepared according to package instructions*); Food coloring of your choice; Sprinkles; *Note - keep the cake batters in 2 separate bowls. One will be colored, the other stays white; 1/2 cup vegetable shortening; 1/2 cup unsalted butter (softened); 1 teaspoon vanilla; 4 cups powdered sugar; 2 tablespoons milk (or more for thinner consistency)