01 - Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper.
02 - Whisk together raspberry jam, lemon juice, and cornstarch in a small bowl until cornstarch dissolves completely and mixture is smooth.
03 - Roll pie dough on lightly floured surface to 1/8-inch thickness. Cut 16 heart shapes using a 3.5-inch heart cutter, rerolling scraps as needed.
04 - Arrange 8 heart cutouts on prepared baking sheet, spacing evenly. Place 1 tablespoon jam filling in center of each heart, leaving 1/2-inch border around edges.
05 - Lightly brush dough borders with water. Top with remaining hearts, pressing edges firmly to seal. Crimp edges with fork tines to create decorative seal.
06 - Whisk egg with milk in small bowl. Brush tops of pies with egg wash using pastry brush. Sprinkle generously with granulated sugar.
07 - Cut small X or slit in top of each pie with sharp paring knife to allow steam escape during baking.
08 - Bake 16 to 18 minutes until crust is deep golden brown and filling bubbles slightly. Cool on baking sheet 5 minutes, then transfer to wire rack.