Preheat oven to 350°. Cream peanut butter and sugar until light and fluffy. Beat in egg.
Roll into 1-in. balls. Place 2 in. apart on ungreased baking sheets. Flatten slightly with the bottom of a glass. Bake until tops are slightly cracked, 15-17 minutes. Cool for 3 minutes before removing from pans to wire racks. Immediately press 1 mini peanut butter cup into center of each cookie. Let stand until set.
In a microwave, melt chocolate chips and shortening; stir until smooth. Place mixture in a resealable plastic bag; cut a small hole in a corner of bag. Adhere candy eyes to peanut butter cups with melted chocolate mixture. Pipe 8 lines alongside each peanut butter cup to resemble spider legs. Let stand until set.