Cream the butter, peanut butter, granulated sugar, and brown sugar until light and fluffy, 2 to 3 minutes.
Beat in the egg and vanilla until fully combined.
Whisk together the flour, baking soda, and salt, then mix into the wet ingredients just until combined.
Roll the dough into tablespoon-sized balls and coat each in granulated sugar.
Bake at 350°F for 8 to 10 minutes, until the edges are just set.
Immediately press a mini peanut butter cup into the center of each hot cookie.
Cool on the pan for 5 minutes, then transfer to a rack to cool completely.
Melt the chocolate chips with coconut oil in 20-second microwave bursts until smooth.
Pipe four chocolate legs on each side of the peanut butter cup.
Press two candy eyes onto the front of each cup while the chocolate is still wet.
Let set at room temperature for 20 to 30 minutes before storing.