Preheat the oven according to the cake mix directions. Generously spray the heart-shaped pans with baking spray.
Prepare the cake batter as directed on the box. Divide the batter evenly between 4 to 6 heart pans, filling each about halfway.
Bake using the cupcake bake time listed on the box as your starting point. Check for doneness, then add 2 to 3 minutes at a time until the tops are lightly golden and a toothpick inserted in the center comes out clean. Let the cakes cool completely.
To make the frosting, beat the butter until smooth and creamy. Add half of the powdered sugar and mix well. Add the remaining powdered sugar, heavy cream, and vanilla, then beat until fluffy and smooth.
Divide the frosting evenly between two bowls. Tint one light pink and the other hot pink.
Spread the light pink frosting over the top of each cooled cake. Add sprinkles and drizzle with Valentine’s royal icing.
Transfer the hot pink frosting to a piping bag and pipe a decorative border around the edges. Scatter Valentine’s M&Ms along the border.
Serve right away, or snap the lids on the pans and save for later.