Siobhan's Thumbprint Cookies

Position a rack in the upper third of the oven and a second rack in the lower third then preheat to 350°F. Line 2 baking sheets with parchment paper.

In a large bowl, sift together the flour, baking soda, and salt.

In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar and beat on medium speed, scraping the bowl occasionally, until light and fluffy, about 2 minutes. Add the egg and vanilla and beat until fully incorporated, about 1 minute. With the mixer on low, add the flour mixture, and stir until just combined. Shape the dough into a disc, wrap it in plastic, and chill at least 1 hour and up to 24 hours.

Roll the dough into 1-inch balls and arrange on baking sheets, leaving about 3 inches between cookies. Using your thumb or the round end of a small pestle, make a well in the center of each cookie. Using a teaspoon, fill each well with jam, being careful not to overfill the wells. Bake, switching the cookies between the upper and lower racks about halfway through baking, until golden, about 15 minutes. Cool the cookies on baking sheets for 10 minutes before transferring to a wire rack to cool completely. Dust with confectioners' sugar. Continue baking cookies on cooled baking sheets. DO AHEAD: The cookies can be baked ahead and stored, in an airtight container at room temperature, up to 3 days.