1Heat oven to 375°F. In large bowl, beat butter and powdered sugar with electric mixer on low speed until mixed; beat on medium speed until creamy. Add vanilla and egg; beat until mixed, scraping bowl if necessary. On low speed, beat in flour, baking powder and salt until dough forms. Stir in cashews.
2Shape dough into 1-inch balls. On ungreased cookie sheet, place balls 2 inches apart. Press thumb into center of each cookie to make indentation, but do not press all the way to the cookie sheet.
3Bake 7 to 9 minutes or until edges start to brown. Quickly remake indentation with end of wooden spoon handle if necessary. Remove from cookie sheet to cooling rack.
4Place 1 chocolate candy quarter in thumbprint of each warm cookie. Let stand 3 minutes to melt; with tip of small knife, swirl slightly to fill thumbprint. If desired, sprinkle with additional chopped cashews. Cool completely.