Preheat oven to 325°F; line 2 baking sheets with parchment. Beat butter and 2/3 cup sugar until fluffy. Beat in egg yolk and vanilla, then flour and salt. Shape dough into 2 disks; wrap 1 in plastic and refrigerate.
On a floured surface, roll out remaining disk to about a 1/8-inch thickness. Cut out cookies with a 2 1/2-inch heart-shaped cutter. Use a smaller heart cutter to cut a window out of half of cookies. With a spatula, carefully transfer cookies to prepared baking sheets. Repeat with remaining dough. Gather scraps and roll out again to make more cookies.
Bake cookies in batches until edges of cookies are just golden, 10 to 12 minutes. Let cool for 5 minutes, then transfer cookies onto wire racks to let cool completely.
Spread 3/4 tsp. jam over flat side of each cookie that doesn't have a window. Top with remaining cookies, flat side down, so that jam shows through windows. Carefully sift 3 Tbsp. confectioners' sugar over cookies.