Tender-crisp and buttery, these Thumbprint Cookies can be filled with any jam you love. A class at Christmas, they shine like gems on your cookie plate!
In a standing mixer with the paddle attachment at medium speed, or with an electric mixer, beat butter, sugar, egg yolks, and vanilla until smooth.
Gradually add flour and baking powder until combined. Cover and chill at least 30 minutes.
Place rack in middle of oven and preheat to 350 degrees. Line two baking sheets with parchment paper or silicone mats.
Shape dough into 1-inch balls and place on prepared baking sheets 2 inches apart. Using your thumb, make an indentation in the center of each cookie (if the dough is sticky, wet your thumb between cookies).
Bake until light golden brown around the edges, about 11 to 14 minutes, rotating baking sheets after 6 minutes. Remove from oven and immediately transfer to wire cooling racks. Spoon 1/4 teaspoon jam into the indentation on each baked cookie.