Preheat your oven to 350°F. Line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, beat the softened butter and cream cheese together until smooth and creamy. Gradually add the sugar, mixing until well combined.
Add the egg, lemon juice, lemon zest, and vanilla extract to the cream cheese mixture. Mix until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the poppy seeds.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 12-15 minutes, or until the edges are lightly golden. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.