Use an electric mixer to cream butter and sugar together.
Add vanilla and yolk and mix in.
Add flour and baking powder.
If dough seems dry keep mixing until dough forms.
Roll dough out to ¼" thick. I like to roll dough out in between 2 pieces of floured waxed paper.
Cut into 1" circles using a 1" round cookie cutter. Place on cookie sheet.
Bake for 6-8 minutes.
Prepare icing by hand mixing the ingredients together in a small bowl. If it is not thick enough you may need to add a touch more powdered sugar, if it is too thick add a little more milk.
Pipe icing or drop icing onto middle of cooled cookies.
Recipe by THE SEMISWEET SISTERS at https://www.thesemisweetsisters.com/2022/03/29/iced-gem-butter-cookies/