Preheat oven to 375°F (190°C).
Spray a mini muffin tin with nonstick cooking spray.
In a small mixing bowl, mix flour, baking soda, and baking powder. Set aside.
In a large mixing bowl, cream butter and sugar until fluffy. Beat in egg and vanilla.
Gradually add dry ingredients to wet, mixing until combined.
Form 1″ balls of dough and place in the muffin tin. Bake 8-10 minutes until golden.
Press down the center of each cookie to form a cup; let cool.
For the filling, cream butter and sugar. Mix in 2 whole eggs and 2 egg yolks, then lemon juice.
Cook the filling over medium heat, whisking, until thickened, about 5 minutes. Let cool slightly.
Spoon lemon curd into cookie cups. Top with lemon zest and chill until set.
Remove from tin, sprinkle with powdered sugar, and serve.
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