Whisk flour, baking soda, and salt together.
Beat softened butter and peanut butter until smooth.
Add brown sugar and granulated sugar and beat for 1–2 minutes.
Mix in the egg.
Add vanilla.
Mix in the dry ingredients just until combined.
Cover and refrigerate dough for 30 minutes.
Preheat oven to 350°F (175°C) and line baking sheets with parchment.
Portion dough into approximately 30–35 g balls.
Arrange about 2 inches apart.
Bake for 9–11 minutes, until the edges are lightly golden and centers remain soft.
Immediately make a shallow indentation in the center of each cookie.
Cool on the baking sheet for 3–4 minutes.
Gently press one round chocolate candy into each indentation.
Cool completely.
Melt the semisweet chocolate.
Transfer to a small piping bag and cut a tiny opening.
Pipe four thin bent legs on each side of every chocolate body.
Attach two candy eyes using tiny dots of melted chocolate.
Allow the chocolate to set before serving.