Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Place sugar, peanut butter, and egg in a medium bowl; beat with a mixer at medium speed until smooth. Add flour; stir well. Shape dough into 30 (1-inch) balls; place 1 inch apart on baking sheets coated with cooking spray. Carefully press thumb into center of each cookie, leaving a slight indentation.
Bake cookies at 375° for 12 minutes or until golden. Remove from pans; cool on wire racks. Spoon 1/2 teaspoon jam into the center of each cookie.
Young Chefs can:
Place dough balls on cookie sheet
Indent cookies with thumb
Older Chefs can:
Beat ingredients with a mixer with adult supervision
Roll dough into balls, and spoon jam into indentation