01 - Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper.
02 - Combine raspberry jam, lemon juice, and cornstarch in a small bowl. Stir until cornstarch is completely dissolved and mixture is smooth.
03 - Lightly flour work surface. Roll pie dough to 1/8-inch thickness. Using a 3.5-inch heart-shaped cutter, cut 16 hearts total from both dough sheets.
04 - Arrange 8 heart cutouts on prepared baking sheet. Place approximately 1 tablespoon jam filling in center of each heart, leaving a 1/2-inch border around edges.
05 - Lightly brush edges with water. Top with remaining 8 hearts. Press edges firmly to seal. Crimp edges with fork tines to ensure tight closure.
06 - Whisk egg and milk together in a small bowl until blended. Brush tops of each pie with egg wash using a pastry brush. Sprinkle generously with granulated sugar.
07 - Use a small sharp knife to cut a tiny slit in the top center of each pie to allow steam escape during baking.
08 - Bake for 16 to 18 minutes until tops are golden brown and pastry is crisp. Transfer to wire rack and cool for at least 15 minutes before serving.