Preheat oven to 350 degrees F (175 degrees C). Line baking sheets with parchment paper.
Stir together the flour, poppy seeds, baking powder and salt.
In a medium bowl cream the butter and sugar together until light; beat in the egg yolk, lemon zest and lemon juice. Fold in the flour mixture and mix well.
Divide dough in half and roll each half out on a lightly floured surface until 1/8 to 1/4 inch thick.
Cut with cookie cutters and place cookies on the prepared baking sheet.
Brush tops of cookies with beaten egg white and sprinkle with white sugar.
Bake at 350 degrees F (175 degrees C) for 12 to 15 minutes or until golden, the edges should be light brown.
