In a stand mixer, beat the softened butter along with light brown sugar and granulated sugar until fluffy. Add creamy peanut butter, eggs, and vanilla extract, mixing until fully combined.

In a separate bowl, whisk together all-purpose flour, corn starch, baking soda, and salt. Gradually add this mixture to the wet ingredients, stirring until a soft dough forms.

Wrap the dough in plastic wrap and refrigerate for 30-60 minutes.

While the dough chills, preheat your oven to 375°F (190°C) and line your cookie sheets with parchment paper.

Roll the dough into 1.5-tablespoon balls, then coat them lightly in granulated sugar. Place the balls on the prepared cookie sheets.

Bake for 8-10 minutes, then gently indent the warm cookies with chilled Reese's.

Microwave mini marshmallows in small batches until they puff up. Carefully stretch and pull them over the cooled cookies.

Melt dark chocolate wafers in the microwave and use them to secure the Reese's onto each cookie. Pipe chocolate legs and attach small candy eyes.

Allow decorations to set before serving.