Cream butter; gradually add sugar, beating well. Add egg and vanilla, mixing well. Stir in peanut butter.
Combine flour and baking powder; add to peanut butter mixture, beating well. Stir in pecans. Shape into 1-inch balls; place 2 inches apart on ungreased baking sheets. Dip a fork in water, and flatten balls to 1/2-inch thickness.
Bake at 350° for 10 minutes or until lightly browned. Remove to wire racks, and cool.