Preparation

• Position a rack in the center of the oven and heat the oven to 300°F. Lightly butter an 8-inch-square baking dish.
• In a medium bowl, stir the butter, sugars, and poppyseeds with a silicone spatula or wooden spoon until creamy. Gradually add the flour, 1/4 cup at a time, until combined. Add about 3 tsp. water, 1 tsp. at a time, just until a soft, tacky dough forms. Do not overmix. Using your fingers, press the dough into the baking dish (don’t worry if it isn’t completely even).
• Bake until pale golden in the center and slightly darker around the edges, 50 to 60 minutes. Transfer to a rack and let sit until cool enough to handle. Run a knife around the edge of the baking dish to loosen the shortbread, then gently invert to remove it. Score with a knife or bench scraper at 1/2-inch intervals to make 1/2-inch squares. When completely cooled, cut through.

Make Ahead Tips

The cookies can be stored in an airtight container at room temperature for up to 5 days.