Preheat your oven to 350°F. Line 2 baking sheets with parchment and set aside.
Start by making your cookie dough. Combine the softened butter, brown sugar, and white granulated sugar in a large mixing bowl. Whisk until smooth and creamy.
Add the egg and whisk for about 1-2 minutes, or until the mixture develops a fluffy texture.
Slowly whisk in the flour, baking powder, baking soda, poppy seeds and lemon zest until no dry pockets remain.
Use a standard large size cookie scoop to drop balls of cookie dough onto a baking sheet. Space your cookies about 2 inches apart and bake them for 11 minutes.
Let your cookies cool on the pan for a few minutes and then move them to a wire cooling rack. Let your cookies cool for an hour on the counter.
Once your cookies are cooled, prepare your frosting by whisking together the butter, powdered sugar, and lemon juice. Use a spatula or piping bag to frost your cookies. Enjoy!
Notes
Store leftovers in an airtight container on the counter for up to 5 days. Cookies can be frozen for up to 2 months.