Beat butter, sugar and peanut butter in medium bowl until creamy. Add egg and vanilla; beat well. Stir together flour, baking soda and salt; add to butter mixture, blend well. Stir in Reese's pieces. Drop by rounded teaspoons onto ungreased cookies sheets.
Bake 15 minutes or until light golden brown around the edges. Cool 1 minutes on cookie sheets. Remove to wire rack; cool completely.