Preheat the oven to 350 and prepare a cookie sheet with parchment paper. Set to the side.
Prepare the lemon by juicing and zesting it. Place the zest and the juice in separate bowls and set to the side.
Melt the butter in the microwave. Allow the butter to cool to room temperature.
Combine the flour, baking powder, baking soda, and salt in a medium bowl. Set to the side.
In a large bowl, whisk together the brown sugar, granulated sugar, and cooled melted butter. Then, add in the eggs, lemon zest, lemon juice, and vanilla. Mix until combined.
Fold the poppy seeds into the dough using a rubber spatula.
Scoop the cookies onto the prepared cookie sheet, about 6 at a time.
Bake the cookies in the oven for 11-12 minutes, until the edges are firm.
Let the cookies cool slightly on the cookie sheet before moving to a cooling rack to continue cooling.