Bring dough to room temperature. Place between two 12-by-14-inch pieces of parchment; roll out to an 8-by-12-inch rectangle. Transfer to a baking sheet; chill 30 minutes.
Transfer dough to a clean work surface. Remove top piece of parchment; sprinkle poppy seeds over top. Using bottom parchment to support dough, fold bottom edge to meet middle. Fold over two more times, forming a 1-inch-tall rectangle. Wrap; chill at least 2 hours.
Heat oven to 350 degrees. Line two baking sheets with parchment. Remove parchment from dough, and cut into 1/4-inch-thick squares. Place on baking sheets, spaced 2 inches apart.
Bake cookies until edges turn light golden, about 15 minutes. Transfer to a wire rack to cool. Bake or freeze remaining dough. Store in an airtight container up to 2 weeks.