In a mixing bowl, cream the shortening, peanut butter and brown sugar until light and fluffy. Beat in eggs, one at a time, beating well after each addition. Add vanilla. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk, beating well after each addition.
Fill paper-lined muffin cups with 1/4 cup of batter. Press a peanut butter cup into the center of each until top edge is even with batter. Bake at 350°F for 22 to 24 minutes or until a toothpick inserted on an angle toward the center of the cupcakes comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.