1. Add melted-and-cooled butter to a large bowl. Add sugar and whisk until totally combined. It should be thick and shiny. Whisk in egg, vanilla extract, and lemon juice until combined and smooth, about 60 seconds.

2. Add the flour, baking powder, cornstarch, lemon zest, and salt into the bowl with the other ingredients and use a spatula to fold the dry ingredients into the wet ingredients until just combined. Dough will be soft.

3. Chill dough for 2 hours.

4. Line a baking sheet with parchment paper. Preheat oven to 350℉.

5. Divide dough into 6 large 1/3 cup cookies (use a scoop or a measuring cup). Shape each cookie into a disk about 2.5” in diameter and 7/8” tall.

6. Space cookies 3” apart and bake at 350℉ for 15-16 minutes, or until puffed and lightly golden on edges. Don’t overbake.

7. Allow to cool completely before decorating.

8. In a medium bowl, use an electric mixer with the beater attachments to beat the softened butter on medium speed until creamy and smooth.

9. Turn mixer to low and gradually add the powdered sugar to the butter. Beat in each addition until combined.

10. In a small bowl, whisk the milk (or cream), lemonade mix, vanilla extract, lemon juice, and lemon zest until the lemonade mix is totally dissolved.

11. Add this mixture to the buttercream and beat to combine. Optionally, add a tiny bit of yellow food coloring for a brighter color.

12. Use 1-2 tablespoons of heavy cream or milk to adjust the consistency of the buttercream.

13. Once the frosting is smooth, bring the mixer up to medium-high speed for 1-2 minutes until it’s light and fluffy.

14. Frost each cooled cookie with a very thick layer of frosting using a 3-tablespoon scoop and smooth it with a spatula.

15. Optionally, top with a fresh lemon slice and enjoy!