Heat oven to 350° F (177° C). Line a small baking sheet with parchment paper or a silicone baking mat.
With an electric hand mixer, beat the butter and sugar until light and fluffy, about 1 minute.
Mix in the egg yolk and vanilla until well combined.
Gently mix in the flour.
Scoop 1 1/2 tablespoon portions onto a baking sheet and roll each into a smooth ball.
Press the center of each ball with your thumb or the back of a 1 teaspoon measuring spoon.
Spoon about 1/2 teaspoon of jam into each indentation.
Bake for 8 to 10 minutes, until set and lightly golden.
Notes
Read the Recipe: Go through the recipe and ingredient notes before starting. These cookies come together quickly, so have everything measured and ready.
Sugar Coating (Optional): Roll dough balls in granulated sugar before baking for extra sweetness and texture.
Measure Flour Properly: Spoon flour into a dry measuring cup and level with a knife. Avoid scooping from the bag, which packs the flour and can make cookies dense.
Room Temperature Jam: Soft jam is easier to add and won’t firm up the dough.