Preheat oven to 350 degrees.
Place 12 of the mini pie shells on a baking sheet. (If using frozen pie shells thaw them for about 15 minutes.)
In a large bowl add frozen raspberries and stir in the lemon juice. Then, coat the raspberry mixture with sugar and cornstarch.
Scoop prepared raspberries into the mini pie crust shells without overfilling them.
Bake for 20 to 25 minutes or until crust is golden. Cool on a rack.
Take the extra mini pie crust shells and roll each one out on a smooth surface. Roll them just enough to make it flat but keep its shape and size.
Use a heart shaped cookie cutter to cut out small hearts. Place them on baking sheet lined with parchment paper and bake for 10 minutes.
Once the heart crusts are baked, place in the center of each one of the tarts and allow to cool.