1. Preheat the oven to slow (300°).
2. Combine the peanut butter, brown sugar, egg and vanilla in a large bowl; stir until smooth. Roll the dough into small balls, about 3/4 inch in diameter. Place the balls, about 1 inch apart, on an ungreased cookie sheet. Flatten each slightly. Press 1 chocolate piece in the center of each cookie.
3. Bake the cookies in the preheated slow oven (300°) for 20 minutes or until light brown. The kisses can be stored in airtight containers for several days.