Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
In a medium bowl, whisk together flour, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, brown sugar, and peanut butter until light and fluffy. Beat in the egg and vanilla extract.
Gradually add the dry ingredients to the wet mixture until just combined.
Scoop tablespoon-sized portions of dough, roll into balls, and place on the prepared baking sheets about 2 inches apart.
Flatten slightly with the palm of your hand or the bottom of a glass. Bake for 9–11 minutes, or until lightly golden and set.
Remove from the oven and immediately press a chocolate truffle into the center of each cookie while still warm. Allow the cookies to cool completely so the chocolate sets.
Melt the semisweet chocolate chips in a microwave-safe bowl in 20-second increments, stirring until smooth.
Transfer the melted chocolate to a piping bag or small zip-top bag with a corner snipped off.
Pipe 4 legs on each side of the chocolate “body.” Attach candy eyes using a tiny dab of melted chocolate as glue.
Let the decorations set fully before serving or storing.