Make heart cupcakes
Follow the instructions on your pink velvet cake mix to make pink cake hearts using your well greased mini heart mold. Keep in mind that the heart cupcakes are small, so they will bake in half the time a normal cupcake would. Adjust your baking time accordingly. Any leftover cake batter can be used to make additional cupcakes.
Let cool & cut off tops
After baking the cupcakes allow to cool, then cut off the tops of each heart cupcake before removing it from the mold. Set the tops aside to make cake macaroons. Remove the hearts from the mold and set aside as well.
Make white cake
Follow the instructions to make your white cake mix. Pour the batter into the brownie bar pan, but don't fill the wells quite to the top.
Sink hearts in cake batter
Sink the heart shaped cake pieces into the white cake batter, all precisely in the middle and facing the same direction, so you know which direction to cut them open later. Don't worry if the tops show, the batter expands while baking and will cover them.
Remove & trim cakes
Bake the white cupcakes and allow them to cool a few minutes before removing them from the pan. As you remove them from the pan, mark which side of the cake is the front by scraping it with a butter knife. If the top overflowed into a muffin top, cut the edges off to create a perfect square. Don't cut the top off, otherwise you'll cut the heart, just trim the sides.
Decorate the cakes as desired. Keep track of which side is the front so you can cut into it and reveal the heart later by decorating the front differently or piping a heart on top facing forward.
Top with heart shaped cake pieces
Embellish the top of your Mini Valentine Cake with the extra heart shaped cake pieces. Be sure to frost them as well so they stay nice and moist. You can even decorate your mini cakes with sprinkles, decorating gel, fruit or your favorite tasty embellishment.
Plate your cakes and use decorating gel to write a little note beside your cake.
Cut to reveal heart
Just before biting into their cake, cut it down the middle to reveal the special heart surprise inside.