1. Preheat oven to 375 degrees and line muffin tin with paper cups.
2. Put peanut butter, shortening, and vanilla in mixing bowl and cream at medium high speed with mixer. Add brown sugar gradually with mixer running until light and fluffy. Add one egg, beating well, then add other egg, beating until combined.
3. Mix together the flour, baking powder, and salt.
4. With mixer running, add a little of the flour mixture, then a little of the milk, then the flour, then the milk, until it is all incorporated into batter.
5. Pour batter into muffin cups, filling 1/2 full.
6. Bake in preheated oven for 18 to 20 minutes or until cupcakes are firm. Remove and cool on rack.
7. Put about 1 cup of peanut butter in a bowl and add a little powdered sugar; mix and add more powdered sugar until you get the texture and taste of frosting. The amount of peanut butter and powdered sugar depends on how thick you want the frosting on the cupcake. Spread this frosting on the cupcakes and enjoy!