These pretty little cookies are a hit at kids’ parties because the children like to roll and fill them.
Preheat the oven to 350 degrees. Lightly spray 2 baking sheets with flavorless vegetable oil spray or use nonstick baking sheets.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment and set on medium-high speed, beat the butter and granulated sugar until light and fluffy. Add the egg and beat until well mixed.
Slowly add the dry ingredients, mixing on medium-low speed, until just incorporated. Add the milk and lemon extract until the dough comes together.
Turn the dough out onto a lightly floured work surface and divide into 2 pieces. Using a lightly floured rolling pin, roll each piece about 1/8 inch thick. Using a 2- to 2 1/2-inch-wide-heart-shape cookie cutter, stamp out heart-shape cookies. Gather the dough scraps, reroll, and cut out more cookies.
Put the cookies on the baking sheets and bake for 10 to 12 minutes, or until very lightly browned and just beginning to crisp around the edges. Remove the baking sheets from the oven and let the cookies sit on them for 2 to 3 minutes before removing with a spatula. Cool the cookies on wire racks.
When the cookies are cool, spread half of them with jam and sandwich with the rest. Dust the cookies with confectioners’ sugar. The cookies keep in an airtight container for up to 2 days.