Poppy seeds give these cookies their distinctive crunch.
Adjust rack to lower third of oven and preheat oven to 350 degrees F. Line baking sheets with baking parchment. Place flour, poppy seeds, sugar, and salt into a food processor, and process just to blend ingredients. Add the butter in pieces, processing with on/off bursts until mixture has the consistency of cornmeal. In a small bowl, whisk the egg and lemon extract. With the processor motor running, pour egg mixture down the feed tube, and process just until ingredients form a ball.
Measuring 1 level teaspoonful at a time, shape dough into balls and set ½ inch apart onto the baking sheets. Gently make an indentation in the center of each ball, pressing your thumb down almost to the bottom without breaking through the dough. Bake for 10 minutes, or until the cookies are ivory-colored on top and pale golden on the bottom. Transfer cookies to a wire rack to cool. Place ¼ teaspoonful of strawberry preserves into each indentation.