Preheat the oven to 375°F (190.6°C).
Start making the lemon butter cookies by adding your soft butter into the mixing bowl. Beat on medium speed until your butter is a creamy texture.
Add in the sugar, powdered sugar, and corn starch. Continue beating until it is mixed. Finally, add your freshly squeezed lemon juice, and mix until combined.
Add in all-purpose flour, baking powder, and salt. Mix until your dry ingredients are combined with your wet mixture.
When your cookie dough is mixed thoroughly, place them into the fridge and allow them to chill for 1 hour.
After they have chilled, roll them into balls, and place them onto a lightly greased cookie sheet.
Bake for 8 to 10 minutes, or just until the edges begin to turn a light golden color. Then remove the cookies from the oven. Use a round spoon or use a tablespoon to press the center of each cookie down.
Let the cookies cool to room temperature before filling them.
When the cookies have cooled, you can add the filling into the center of the cookies.