1. Preheat oven to 325, 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.
2. On a floured surface, roll out remaining disk to about ¼ in thickness. Cut out cookies with a 2 ½ in heat shape cutter. Use a smaller heart cutter to cut a window out half the cookies. With a spatula, carefully transfer cookies to prepared baking sheets. Repeat with remaining dough.
3. Bake cookies in batches until edges of cookies are just golden, 10 to 12 mins. Let cool for 5 mins, then transfer cookies onto wire racks to let cool completely.
4. Spear ¾ 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. sugar over the cookies.
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