Preheat your oven to 350°F and line two baking sheets with parchment paper.
In a large bowl, beat the butter, peanut butter, and both sugars until light and fluffy, about 2 minutes.
Add the egg and vanilla extract, beating until well combined.
Whisk the flour, baking soda, and salt together in a separate bowl, then gradually mix into the wet ingredients.
Roll the dough into small balls, roll in granulated sugar, and place on prepared sheets.
Bake for 11 minutes until edges are set but the center remains soft.
Immediately upon removing from the oven, press a mini peanut butter cup into the center of each cookie.
Let the cookies cool completely.
In a microwave-safe bowl, melt the chocolate chips and coconut oil together in 20-second increments until smooth.
Transfer melted chocolate to a piping bag or small plastic bag with the corner snipped, pipe spider legs onto the cookies, and attach the candy eyes.