Preheat the oven to 350°F. Line two rimmed baking sheet with parchment and set aside.
Whisk together the flour, cocoa, baking soda, and salt. Set aside.
Microwave the butter until very soft and slightly melted. Add the granulated and brown sugars. Use an electric mixer to combine until the sugar begins to dissolve. Add the egg and vanilla and continue to mix until smooth.
Stop the mixer and add the flour mixture. Stir with a rubber spatula or wooden spoon until combined. Add the chocolate chips, pecans, and half of the marshmallows. Stir to combine.
Use a mechanical cookie scoop to make dough balls, 3 tablespoons each. Poke any visible marshmallows into the the center of the dough. Place on the baking sheet two inches apart.
Top the dough balls with 3 to 4 mini marshmallows.
Bake for about 10 minutes, or until the edges are set and the centers are soft.
Cool the cookies on their baking sheets on top of a wire rack for 10 minutes before moving with a flat spatula.