The translucent stained glass in these holiday cookies is melted butterscotch candies. Be sure to bake the cookies on parchment paper so the can easily be removed from the cookie sheets.
Cream butter and 1/2 cup of the sugar in a large bowl with an electric mixer on medium high until fluffy. Add egg yolk and vanilla; beat until incorporated, scraping down side of bowl as needed. Add flour and mix on low speed until dough comes together in a ball.
Divide dough into 3 pieces. Wrap each in plastic wrap; refrigerate for at least 2 hours and up to 2 days. (Dough can be wrapped tightly in plastic wrap and frozen for up to 1 month; defrost in refrigerator before using.)
Heat oven to 375 F. Line baking sheets with parchment paper.
Process candies in a food processor until finely ground. Transfer to a small bowl.
Remove one piece of dough from the refrigerator and knead it 4 or 5 times on a lightly floured work surface to soften. With a lightly floured rolling pin, roll out dough 1/8-inch thick. Cut out dough with a 3-inch star cookie cutter. Cut out center of each cookie with a 1-1/4-inch star cookie cutter. Transfer to prepared baking sheets. Refrigerate scraps. (Cutout cookies can be frozen on a baking sheet in layers separated by parchment paper; wrap tightly in plastic. Freeze for up to 1 month. Remove from the freezer, place in single layers on parchment-paper-lined baking sheets and proceed.)
Using a very small spoon, carefully fill each star cutout with candy so it is resting on the parchment paper and is level with the dough (1/4 to 1/2 teaspoon in each). Sprinkle cookies with some of the remaining 1/4 cup sugar. Bake at 375 F until cookies are firm and golden, about 8 minutes (a minute or two longer for frozen dough). Cool completely on the baking sheets. Repeat with the remaining dough, then chilled scraps, using fresh parchment each time. Keep at room temperature in an airtight container for several days.