Preheat the oven to 350°F (convection). Line a baking sheet with parchment paper and set aside.

In a large bowl or stand mixer fitted with the whisk or paddle attachment, beat the room-temperature butter, granulated sugar, and brown sugar until light and off-white in color, about 1–2 minutes.

Add the egg, vanilla extract, corn syrup, and molasses. Mix until fully combined and smooth.

Add the all-purpose flour, baking powder, baking soda, and cocoa powder to the bowl. Gently fold the dry ingredients into the dough until no dry spots remain.

Scoop ⅓ cup portions of cookie dough and place them onto the prepared baking sheet, spacing evenly apart.

Bake for 10–11 minutes, until the edges are set and the centers look slightly underbaked.

For perfectly round cookies, use a round bowl, cup, or cookie cutter to gently swirl around the cookies immediately after they come out of the oven.

Allow the cookies to rest on the baking sheet for about 10 minutes before transferring.

Place the marshmallows on a parchment-lined baking sheet and toast in the oven until the tops are golden and browned. (A kitchen torch works great here too!)

In a mixing bowl, whip the butter on high speed for 3–4 minutes until light and fluffy. Add the powdered sugar and mix until smooth. Pour in the heavy cream and whip on high speed for about 2 minutes. Finally, mix in the vanilla extract and toasted marshmallows until combined.

Transfer the frosting to a piping bag and frost the cooled cookies. Top with extra toasted marshmallows, if desired.

Enjoy fresh, or store cookies in an airtight container in the refrigerator for up to 8 days.