Line twenty-four 2-1/2-inch muffin cups with paper bake cups. Set aside. In a large mixing bowl beat together the peanut butter, shortening, and vanilla on medium to high speed for 30 seconds. Gradually add brown sugar, beating until light and fluffy. Add eggs, one at a time, beating well after each addition.
Stir together flour, baking powder, and salt. Alternately add flour mixture and milk to peanut butter mixture. Fill bake cups half full.
Bake cupcakes in a 375 degree F oven about 20 minutes or until a toothpick inserted in the center of a cupcake comes out clean. Remove from muffin cups and cool on a wire rack. If desired, sift powdered sugar over each or frost with peanut butter. Makes about 24 cupcakes.