Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper.

In a large bowl, cream together the softened unsalted butter and granulated sugar with a handheld mixer until the mixture is light and fluffy, about 2-3 minutes. Add the large egg, vanilla extract, and lemon juice, mixing until everything is fully incorporated and smooth.

Add the all-purpose flour, baking powder, kosher salt, and poppy seeds to the wet ingredients. Gently fold the dry ingredients into the wet mixture by hand until just combined and no streaks of flour remain. Be careful not to overmix.

Scoop the dough and form it into 1-inch balls. Place them about 2 inches apart on the prepared baking sheets.

Using the bottom of a glass or a flat-bottomed measuring cup, gently press down on each cookie dough ball to slightly flatten it.

Bake in the preheated oven for 7-9 minutes, or until the edges of the cookies are a light golden brown. The centers may still look slightly soft.

Allow the cookies to rest on the baking sheets for 10 minutes after baking. This helps them set up. Then, carefully transfer them to a wire rack to cool completely.