In a large bowl, cream together the peanut butter, butter, white sugar, and brown sugar until well blended. Beat in the egg, milk, and vanilla one at a time. Combine the flour, baking powder, and salt; stir into creamed mixture. Roll tablespoonfuls of dough into balls. Place cookies 2 inches apart onto ungreased cookie sheets. Press each ball once with fork tines.
Bake for 8 to 10 minutes in the preheated oven, or until edges are lightly browned.
