• Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper for easy cleanup.
• In a large mixing bowl, using a hand mixer or a stand mixer with a paddle attachment, cream the butter with both sugars on medium speed until light and fluffy, around 1-2 minutes. Add the egg and vanilla extract, blending until fully incorporated and scraping down the sides as needed.
• In a separate bowl, sift together the cocoa powder, flour, baking soda, cornstarch, and salt. Gradually add this dry mixture to the butter mixture on low speed, mixing until just combined. Fold in ⅔ cup of chocolate chips and ⅓ cup of chopped nuts, ensuring they are evenly distributed throughout the dough.
• Portion out the dough using a 3-tablespoon cookie scoop (about 2 ounces per portion). Flatten each portion of dough in the palm of your hand, placing one marshmallow in the center. Wrap the dough around the marshmallow, leaving a bit of marshmallow peeking through the top. If the dough is too sticky or soft to handle, refrigerate it for 30-60 minutes to firm it up slightly.
• Arrange the prepared dough balls on the parchment-lined baking sheets, spacing them a couple of inches apart. Bake for 10-12 minutes, until the cookies have spread and the marshmallow centers are soft. The cookies should look slightly underbaked when removed from the oven, as they will firm up as they cool. Once out of the oven, immediately place the baking sheets on wire racks.

To achieve perfectly round cookies:

• Option 1: Right after removing the cookies from the oven, take a round glass, bowl, or larger cookie cutter and place it over the top of each cookie. Gently swirl in a circular motion to nudge the edges inward, creating a perfect circle. This only works while the cookies are hot and on the baking sheet.
• Option 2: Alternatively, use the back of a spoon to carefully push any uneven edges inward, giving the cookies a neat, round appearance.
• While the cookies are still warm, sprinkle additional chocolate chips and chopped nuts on top. The warmth from the cookies will help these toppings adhere, adding texture and visual appeal. Allow the cookies to cool completely on the wire racks. Enjoy!