Preheat your oven to 350°F (175°C). Grease a muffin tin or line it with cupcake liners.
In a mixing bowl, cream together the butter and granulated sugar until light and fluffy. Gradually add flour, baking powder, and lemon zest; mix until combined.
Press small portions of dough into each muffin tin cup, creating an indentation in the center. Bake for about 10 minutes or until lightly golden.
While the cookie cups bake, whisk together lemon juice, lemon zest, powdered sugar, and egg until smooth.
Once baked cups have cooled slightly, pour in the lemon filling and return to the oven for another 15 minutes or until set.
Cool completely before dusting with powdered sugar. Serve at room temperature or chilled.
