"I started making these jam-filled half-moon cookies in college to keep me energized while I studied, and I still make them as a special treat for my friends every holiday. They are a bit labor-intensive, but very much worth the work."
Beat butter and cream cheese together in a bowl using an electric mixer until smooth and creamy. Stir 1/2 the flour into creamed butter mixture until incorporated; mix in remaining flour until dough is just combined. Refrigerate dough until chilled, at least 1 hour.
Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil.
Stir jam and cornstarch together in a small bowl until cornstarch is dissolved and jam is slightly thickened.
Turn dough onto a floured work surface and roll into 1/4-inch thickness. Cut dough into 3-inch circles using a cookie cutter or the rim of a coffee mug. Spoon about 1 teaspoon jam down the center of each circle. Fold dough circles in half and crimp edges together using a fork. Arrange folded dough on the prepared baking sheet.
Bake in the preheated oven until cookies are golden brown, about 15 minutes.