Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a bowl, whisk together the flour, baking soda, salt, and poppy seeds.
In a separate large bowl, cream the butter and sugar until light and fluffy.
Beat in the egg, lemon zest, lemon juice, and vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Scoop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them 2 inches apart.
Bake for 10-12 minutes, or until the edges are lightly golden.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.