3 cups all-purpose flour (spooned and leveled), plus more for rolling
3/4 teaspoon baking powder
1/4 teaspoon fine salt
1 cup (2 sticks) unsalted butter, room temperature
1 1/4 cups sugar
4 large egg yolks
1 tablespoon pure vanilla extract
Very finely crushed hard candy, such as Jolly Rancher, in various colors (1 cup total)
In a medium bowl, whisk together flour, baking powder, and salt. In a large bowl, using an electric mixer, beat butter and sugar on medium-high until light and fluffy, 3 minutes. Add egg yolks and vanilla and beat to combine. With mixer on low, gradually add flour mixture and beat to combine. Form dough into 2 disks, wrap in plastic, and refrigerate 30 minutes.
Preheat oven to 350 degrees, with racks in upper and lower thirds. Working with 1 disk at a time, roll out dough between floured parchment paper to 1/8-inch thickness. Stack dough in parchment on a baking sheet; refrigerate until firm, 30 minutes. With a 2 1/2-inch Christmas-ball-shaped cutter, cut out dough (reroll scraps, if desired). Place cookies, 1 inch apart, on two parchment-lined baking sheets. With a 1-inch star cutter, cut out dough in center of each cookie. Bake until cookies are pale but set, 8 to 10 minutes, rotating sheets halfway through. Remove sheets from oven; fill cutouts with crushed candy. Bake until cookies begin to brown at edges and candy is melted, 3 minutes. Let cool completely on sheets on wire racks.
Quick Tip: To crush the hard candy, seal in a zip-top bag and pound with a meat mallet or small skillet.
Disks of dough can be refrigerated overnight or frozen, up to 1 month, and thawed in the refrigerator. Let sit at room temperature until soft enough to roll. Cookies can be stored in airtight containers, up to 1 week.