Preheat oven to 175°C (350°F). In a large bowl, cream butter and sugar until light and fluffy. Beat in egg and vanilla. Add flour, baking powder, and salt; mix until a soft dough forms.
Grease a 12-cup muffin tin. Roll dough into 1-inch balls and press into each cup to form small wells. Chill for 10 minutes to prevent spreading.
Bake for 10–12 minutes, until edges are lightly golden. Remove from oven and immediately press the centers down gently using the back of a spoon. Let cool completely.
Whisk sugar, eggs, lemon zest, juice, flour, and melted butter in a saucepan. Cook over low heat, stirring constantly, until thickened (about 5–7 minutes). Cool slightly.
Spoon or pipe the lemon filling into cooled cookie cups. Dust with powdered sugar and garnish with zest or lemon slices.
Notes
Use fresh lemon juice for that real citrus punch.
Store in the refrigerator for up to 4 days.
Add a hint of coconut or almond extract for a unique flavor twist.