Preheat oven to 375 degrees F.
Spray a mini muffin tin with nonstick cooking spray.
In a small mixing bowl, combine flour, baking soda, and baking powder; set aside.
In a large mixing bowl, cream together butter and sugar until fluffy.
Beat in egg and vanilla.
Gradually beat in dry ingredients until combined.
Roll into 1" sized balls and place in muffin tin.
Bake for 8-10 minutes or until golden brown.
Press the insides of the cookies down with the backside of a spoon to make an indent for filling; let cool.
To make the filling, cream together 6 Tablespoons softened butter with 1 cup sugar.
Mix in 2 eggs and just the yolks from the other 2 eggs, and beat until well combined.
Add lemon juice and mix until combined, there will still be some lumps.
Pour filling mixture into a saucepan over medium heat.
Whisk constantly for 5 minutes, or until smooth and thickened.
Remove lemon curd from heat and let cool for 2-3 minutes.
Carefully spoon a teaspoon of lemon curd into each cookie cup.
Top cookie cups with lemon zest of one lemon, and place in fridge for 30 minutes, or until lemon curd has set up.
Remove cookie cups from pan using a butter knife and sprinkle with powdered sugar before serving.