Preheat your oven to 350°F (175°C). In a mixing bowl, cream together butter and sugar until light and fluffy. Add in egg and vanilla extract, mixing until well combined. Gradually incorporate flour, cocoa powder, and baking soda until a dough forms.

Scoop out tablespoon-sized portions of dough, roll them into balls, and place them on a greased or parchment-lined baking sheet. Flatten each ball to form thick discs. Bake for 10-12 minutes until edges are firm and centers are set. Cool on baking sheet for a few minutes before transferring to a wire rack.

While the cookies cool, unwrap peanut butter cups. Melt chocolate in a microwave-safe bowl in 30-second intervals until smooth. Dip the bottom of each peanut butter cup into melted chocolate and place in the center of a cookie.

Break pretzel sticks into smaller segments for legs. Insert four pretzel pieces on each side of the peanut butter cup to create a spider shape.

Add candy eyes to the peanut butter cups using melted chocolate to secure them in place. Allow chocolate to set for about 15 minutes before serving.