For the Cake:
Line the bottom of two cake pans with parchment paper, or spray well with non-stick baking spray. Bake cake according to package directions, splitting the batter into the two pans equally. Cool completely in the refrigerator or freezer.
For the Frosting:
Once the cakes are completely cooled, carefully remove them from the cake pan. Lay one cake heart down on your work surface, and apply the half of the frosting to the top, spreading to coat evenly. Flip the second cake heart upside down and carefully place on top of the first cake heart. Use the remainder of the frosting to lightly coat the top of the cake, as well as the sides.
Dust a clean work surface with cornstarch or powdered sugar. Knead the fondant for a minute or so until it's soft and pliable. If you're going to add color to your fondant, do so now, and if you'll be using fondant for added decoration, make sure you reserve a little bit for that. If needed, dust your surface with more cornstarch or powdered sugar. Roll the fondant to 1/8" thick. Carefully lift it from your work surface and place it on top of the cake, using your hands to mold it to the shape of the cake. Trim the excess with a sharp knife.
If making decorations for the top of the cake, knead a small amount of food coloring (careful - a little bit goes a long way!) into the fondant and roll using the same procedure. Cut into shapes with cookie cutters or a sharp knife, and secure your decorations on to the cake with a small brush of water.