Preheat oven to 180 degrees C. Line baking trays with baking 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 5mm thick. Cut with biscuit cutters and place biscuits on the prepared baking tray. Brush tops of biscuits with beaten egg white and sprinkle with white sugar.
Bake for 12 to 15 minutes or until golden, the edges should be light brown.