In a large mixing bowl, cream together unsalted butter, light brown sugar, and granulated sugar until light and fluffy.

Incorporate smooth peanut butter, then mix in the egg, vanilla extract, and whole milk until well combined.

In a separate bowl, whisk together flour, baking soda, and salt, then add this mixture to the wet ingredients, blending until smooth.

Cover the dough and refrigerate for at least one hour.

Preheat your oven to 350°F (175°C). Scoop out chilled dough, roll it into balls, and space them on a parchment-lined baking sheet.

Bake for 10-12 minutes, pulling out when edges are set and centers are soft.

Allow cookies to cool for 3-5 minutes before pressing a Reese's Peanut Butter Cup into each cookie.

Once cooled completely, melt semi-sweet chocolate and pipe spider legs onto each cookie.

Finish with candy eyes for decoration.