Blend mint and sugar in a food processor until finely chopped. Beat with butter until creamy; beat in egg and vanilla.

Mix in flour, baking powder and salt. Cover; chill 30 minutes.

Heat oven to 350°F. Line two sheets. Roll dough into 24 balls; flatten slightly and space 2 inches apart.

Bake 10–12 minutes until edges set. Cool on sheets 5 minutes; transfer to a rack.