Prepare the Cookie Cups:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 24-cup mini muffin tin or line it with mini cupcake liners.
2. Cream Butter and Sugar: In a large mixing bowl, beat the butter and sugar together with an electric mixer on medium speed until light and fluffy.
3. Add Egg and Vanilla: Mix in the egg and vanilla extract, scraping down the sides of the bowl as needed.
4. Incorporate Dry Ingredients: Gradually add the flour, baking powder, and salt. Mix until just combined.
5. Form the Cookie Cups: Roll the dough into 1-inch balls and press each into the prepared mini muffin tin, creating a small well in the center with your thumb or a spoon.

Bake the Cookie Cups:

6. Bake: Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
7. Cool: Allow the cookie cups to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

Prepare the Lemon Filling:

8. Whisk Ingredients: In a medium bowl, whisk together the sugar, eggs, lemon juice, lemon zest, melted butter, and flour until smooth.
9. Cook the Filling: Heat the mixture in a small saucepan over medium heat, stirring constantly, until it thickens (about 5 minutes). Remove from heat and let cool slightly.

Assemble the Cookie Cups:

10. Fill the Cups: Spoon the lemon filling into the cooled cookie cups, smoothing the tops with the back of a spoon.
11. Chill: Refrigerate the filled cookie cups for at least 45 minutes to set the filling.

Garnish and Serve:

12. Add Final Touches: Dust the tops with powdered sugar just before serving for an elegant presentation.