Preheat oven to 350 degrees. Use parchment paper or a silpat mat to line a baking sheet.
Cream the butter, sugar and brown sugar in a stand mixer until light and fluffy (about 2-3 minutes). Scrape the sides of the bowl to make sure all ingredients are blended.
Add in the peanut butter, egg, vanilla and milk and beat until combined.
Add in flour, baking soda, baking powder and salt. Stir until all combined.
Scoop out dough and roll into balls. Roll balls in sugar. Place dough balls on prepared pan (12 cookies to a pan if smaller cookies).
Bake for 9-10 minutes.
Immediately after pulling out of the oven, gently press an upside down frozen peanut butter cup on top of each cookie and transfer to a wire rack to cool.
Put in place the candy eyes while chocolate is still slightly soft.
Chill cookies for 10 minutes to allow cookies to become firm.
Melt 1/2 cup chocolate chips in microwave. Place melted chocolate in the corner of a zippered sandwich bag.
Clip a tiny piece off of the corner of the bag to allow for piping and draw eight legs on each spider cookie.
Place in the refrigerator for 10 minutes to allow legs to set. Enjoy.