Preheat the oven to 175°C (350°F). Line a baking sheet with parchment paper.

In a bowl, whisk together flour, baking powder, and salt. Set aside.

In a larger bowl, cream softened butter, sugar, and lemon zest until light and fluffy. Mix in the egg yolk and vanilla extract.

Gradually add the dry ingredients to the wet ingredients, stirring until combined. Fold in the poppy seeds.

Scoop tablespoon-sized portions of dough onto the prepared baking sheet. Flatten slightly with your fingers or the back of a spoon.

Bake for 10-12 minutes, or until the edges are lightly golden. Let the biscuits cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.