Preheat oven to 350 degrees and place 12 of your mini pie shells on a baking sheet. (If pie shells are frozen, be sure to thaw them for about 15 minutes.)
Place frozen raspberries in a large bowl and mix in lemon juice. Then, coat the raspberry mixture with sugar and cornstarch. Scoop into your mini pies without overfilling them.
Bake for 20 to 25 minutes or until crust is golden. Cool on a rack.
Take your spare mini pie shells and roll it on a smooth surface, just enough to make it flat but keep its shape and size. Cut out small hearts with the help of a cookie cutter. Place on your baking sheet and bake for 10 minutes.
Once heart crusts are baked, place in the center of your tarts and allow to cool.