Heat oven to 350°F. Line two baking sheets with parchment. Whisk flour, baking soda, and salt together.

Beat softened butter, peanut butter, brown sugar, and granulated sugar until evenly creamy, about 2 minutes. Beat in egg and vanilla. Stir in the flour mixture just until combined.

Divide into 24 equal portions, about 27 g dough each, and roll into balls. Space 2 inches apart on the sheets and gently flatten to about 1/2 inch thick. If dough is too soft to shape, chill briefly until manageable.

Bake one sheet at a time for 10–12 minutes, until edges are set and lightly golden and the tops are no longer wet. Allow 24 minutes for two batches. Let stand on the sheet for 5 minutes, then transfer to a rack to cool completely.

Melt the chocolate chips in a microwave-safe bowl in 20-second bursts, stirring between each, just until smooth. Spoon into a piping bag with a small opening.

Place a small dot of melted chocolate in each cooled cookie center. Set an unwrapped miniature peanut butter cup upside down on the dot. Pipe four thin legs on each side of the cup. Attach two candy eyes to the front with tiny chocolate dots.

Let the chocolate set fully, about 20 minutes in a cool room. Store in an airtight container in a cool, dry place for up to 3 days for best quality; keep away from heat that melts the decorations.