Preheat the oven to a deliciously warm 350 degrees F (175 degrees C) and prepare your baking sheets by lining them with parchment paper.
Combine the flour, poppy seeds, baking powder, and salt, mixing well.
Combine butter and sugar in a medium bowl and cream together until light and fluffy. Mix in egg yolk, lemon zest, and lemon juice. Gently fold in the flour mixture until well combined.
Separate the dough into two parts and roll each part out on a floured surface until it is 1/8 to 1/4 inch thick. Use cookie cutters to create shapes and transfer the cookies onto a baking sheet. Brush the cookies with beaten egg white and sprinkle with white sugar for extra sweetness.
Bake at 350 degrees F (175 degrees C) for 12 to 15 minutes until golden with light brown edges.