Ingredients
1/2 cup (1 stick) unsalted butter, room temperature
1/2 cup plus 2 tablespoons sugar
1 large egg yolk, plus 1 large egg white, lightly beaten
1 teaspoon pure vanilla extract
1 1/4 cups unbleached all-purpose flour
1/4 teaspoon kosher salt
1/2 cup blanched almonds, finely ground
1/2 cup jam or preserves, such as apricot, red currant, or raspberry, strained if desired
Preparation
Preheat oven to 325 degrees. With an electric mixer on medium speed, beat butter and 1/2 cup sugar until light and fluffy, about 3 minutes. Add egg yolk and vanilla; beat well. In a separate bowl, whisk together flour and salt; add to butter mixture, beating on low until combined.
Combine ground almonds with remaining 2 tablespoons sugar in a small bowl. Form dough into 1-inch balls, and dip in beaten egg white, then in almond-and-sugar mixture. Make a deep indentation in the center of each ball with your finger or a wooden spoon. Place 1 inch apart on parchment-lined baking sheets.
Bake 10 minutes, remove from oven, and press down the centers again. Rotate sheets, and bake until golden brown, 8 to 10 minutes more. Transfer sheets to a wire rack to cool slightly. Fill center of each cookie with about 1 teaspoon jam. Let cool completely. Thumbprints can be stored at room temperature in an airtight container, between parchment, up to 1 day (or 3 days, if not filled with jam).
