Preheat oven and line pans:

Set the oven to the temperature specified on the cake mix package and line two 9x13-inch pans with parchment.

Mix first cake mix:

Mix one cake mix according to package instructions; set aside the other box.

Color the batter:

Add pink or blue gel coloring to the batter and stir until uniform in color.

Bake colored cakes:

Transfer the colored batter into the prepared pans and bake for 16 to 18 minutes; test with a toothpick or knife—it should come out clean.

Cool and cut hearts:

Let the cakes cool fully, then use a heart-shaped cutter to stamp out hearts.

Mix second cake mix:

Prepare the second cake mix per package directions and line a muffin tin with cupcake liners.

Fill liners with batter and hearts:

Spoon batter into each liner until half full, place a cake heart in the middle, then cover with remaining batter until each liner is about three-quarters full.

Bake cupcakes:

Bake the cupcakes for 14 to 16 minutes; a toothpick inserted in the center should come out clean. Allow to cool completely.

Cream butter and vanilla:

While cupcakes are cooling, prepare the frosting: cream the softened butter and vanilla extract together until smooth and creamy.

Add first sugar and cream:

Add 2 cups of powdered sugar plus 2 tablespoons of heavy cream; blend well.

Incorporate remaining sugar:

Gradually incorporate the remaining 2 cups of powdered sugar, mixing thoroughly.

Beat frosting until fluffy:

Increase mixer speed to high and beat for 3 to 5 minutes until the frosting is very light and fluffy. If needed, adjust consistency with extra heavy cream or powdered sugar.

Color piping bag:

For two-tone frosting, carefully streak pink and blue gel coloring up the interior sides of a piping bag.

Pipe frosting on cupcakes:

Fill the piping bag with frosting and pipe onto the cooled cupcakes.