1 cup (2 sticks) unsalted butter, room temperature
2/3 cup granulated sugar
1 large egg yolk
1 teaspoon vanilla extract
2 1/4 cups (280 grams) all-purpose flour
1/2 teaspoon salt
6 ounces (or so) assorted jam
Preheat oven to 375 degrees F. Line a light or medium-colored heavy-weight baking sheet with parchment paper or a silicone baking mat.
Beat together butter and sugar with an electric mixer on medium-high speed until light and fluffy, about 2 to 3 minutes. Beat in egg yolk and vanilla extract. Add flour and salt and mix until incorporated and dough comes together in a ball.
Form dough into 1-inch balls and arrange on prepared baking sheet. Flatten balls slightly with your thumb or the back of a small spoon, leaving an indentation in the center.
Bake cookies for 8 to 10 minutes or until bottoms are just barely golden. Remove baking sheet from oven. If indentations look shallow, further define them with the back of a spoon and then fill each with approximately 1/2 teaspoon jam. Bake for an additional 3 to 4 minutes, or until jam melts slightly and edges of cookies are lightly golden. Transfer cookies to wire racks to cool.