In a large bowl, cream together peanut butter, butter, white sugar and brown sugar until well blended.
Beat in the eggs, one at a time.
Add milk and vanilla.
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 in a criss-cross patter with fork tines.
Bake for 8-10 minutes in the preheated oven, or until edges are lightly browned.