Instructions:
Line a cupcake tin with cupcake papers. Fill them 2/3 full and stick a glass marble between the liner and the actual tin so that an indentation reminiscent of a heart will be baked into the shape. Bake for 15 to 20 minutes. Cool in the pan for 10 minutes, then cool the rest of the way on a cooling rack.
When they're cool, take a paring knife and cut out a little circular plug out of the middle, the size of a nickel or a dime. Cut off the bottom of the plug you've removed and discard it or eat it. This leaves room for you to insert some jelly; do so, and replace the plug on top.
Make your icing (or use store-bought) and dye it pink/red. Ice the cupcakes. If desired, you can reserve a small amount of the frosting and make some dark red and some blue (dyeing the rest the lighter pink), and after icing with pink use a pastry bag to pipe on icing veins so they look like real hearts. (I did not do this in my picture because they were for Valentine's Day, but little kids, especially little boys, like the grossness factor of eating something that looks like a REAL heart that also bleeds when you bite it.)