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

In a stand mixer fitted with a whisk or paddle attachment, beat the room-temperature butter with brown sugar and white sugar for 2 minutes, until light and off-white. Mix in the large egg, vanilla extract, and, if desired, corn syrup and molasses until fully combined.

Add the all-purpose flour, baking soda, baking powder, espresso powder, cocoa powder, hot cocoa mix, and salt. Mix on low speed or fold gently with a spatula just until the dough comes together.

Scoop ⅓-cup portions of cookie dough and place them onto the prepared baking sheet, spacing evenly. Bake for 11–12 minutes, until the edges are set and the centers remain soft. For perfectly round cookies, immediately swirl a round bowl, cup, or cookie cutter around each warm cookie. Let the cookies rest on the baking sheet for about 10 minutes before transferring. Chill cookies in the fridge before adding mousse.

In a mixing bowl, whip together heavy cream, sugar, and hot cocoa mix until stiff peaks form. This can be done by hand with a whisk or using a stand or hand mixer. Chill until ready to use.

Spread 1–2 tablespoons of chocolate mousse on each chilled cookie. Top with mini marshmallows and serve.

Enjoy fresh, or store in the fridge for up to 10 days, or at room temperature for up to 8 days.