In a medium bowl, whisk together the sugar, room temperature butter, vanilla extract, egg, lemon juice, and zest.
In a small bowl, toss together the flour, salt, baking powder, and poppy seeds. Add the dry ingredients to the wet ingredients and fold together until a dough forms. Cover the bowl with plastic wrap and chill the dough for 15 minutes.
Preheat the oven to 350 degrees F and line a baking sheet with parchment paper.
Use a medium cookie scoop to scoop out about 2 tbsp of the chilled dough. Place dough on the prepared baking sheet and repeat, leaving at least 2 inches between each dough ball.
Bake for 12-15 minutes or until the edges are golden brown. Remove from the oven to cool for about 5 minutes on the baking sheet before transferring them to a wire rack to finish cooling and adding the glaze.
Make the glaze by whisking together the powdered sugar, lemon juice, and room temperature mascarpone cheese in a small bowl.
Spoon about 1-2 tsp of glaze on top of each cookie before enjoying!