Preheat oven to 350°. Cream peanut butter and sugar until light and fluffy. Beat in egg and vanilla. , Roll into 1-1/4-in. balls. Place 2 in. apart on ungreased baking sheets. Bake until tops are slightly cracked, 10-12 minutes. Immediately press 1 chocolate kiss into center of each cookie. Cool for 5 minutes before removing from pans to wire racks.