Use a spatula to transfer 8 of the hearts to the baking sheet. Spoon 1 tsp. jam onto the center of each of the 8 hearts. Spread the jam across the dough, leaving a 1/3-inch border. Lightly brush the boarders with milk. Transfer the remaining 8 pieces of pie dough to top the jam-covered hearts, being sure the edges line up. Use the tines of a fork to seal the edges of the dough.