Not everyone is a fan of conversation hearts: In a recent poll, more than two-thirds of Food Network fans said they don’t eat them. But this heart is different — it’s a giant cake! To create the supersize sweet, we turned two cakes into a heart shape, then we covered the whole thing in a homemade marshmallow fondant. For the message, you just need food coloring…and something sweet to say!
Preheat the oven to 350˚ F. Coat a 9-inch round and a 9-inch square cake pan (2 inches deep) with cooking spray. Prepare the cake mixes as directed; divide between the pans. Bake until a toothpick comes out clean, 30 to 35 minutes.
Let the cakes cool, then remove from the pans. Trim the tops of the cakes with a long serrated knife to make them level. Cut the round cake in half as shown.
Working on a cake board (you can cut the board into a heart shape to fit the cake) or a cutting board, position the 2 half circles of cake against 2 sides of the square cake to make a heart.
Cover the cake with the white frosting, using the frosting to fill in any gaps between the cakes.
Combine the marshmallows and water in a large microwave-safe bowl. Microwave, stirring with a rubber spatula, until melted and smooth, 1 to 2 minutes. Add the green food coloring and stir until smooth.
Transfer the marshmallow mixture to a stand mixer. Gradually add all but 1/2 cup of the confectioners' sugar and beat until combined. Beat in the butter.
Scrape the marshmallow fondant onto a sheet of parchment and knead in confectioners' sugar until it’s no longer sticky. Dust the parchment with more sugar; roll out the fondant until it’s large enough to cover the cake, at least 13 by 16 inches.
Carefully flip the parchment and fondant onto the cake; peel off the parchment. Gently press the fondant against the sides of the cake and smooth with your hands; trim the excess. Brush off the extra sugar.
Use the wide end of a chopstick to press letters into the fondant (it’s best to do this before the fondant dries out). Use a small brush to paint hot pink or red food coloring in the letter indentations.