In a medium bowl, vigorously whisk together the all-purpose flour, Dutch-process cocoa powder, baking soda, and fine sea salt until there are no clumps. Set aside.
In a large bowl, whisk the melted unsalted butter with the packed dark brown sugar and granulated sugar until fully combined and no sugar lumps remain. Whisk in the eggs one at a time, followed by the vanilla extract, beating smoothly until the mixture lightens slightly in color. Pour the dry ingredients into the wet mixture. Use a sturdy rubber spatula to fold them together just until a few streaks of flour remain. Do not overmix. Fold in the chocolate chips, chopped toasted nuts, and half of the mini marshmallows. Cover the bowl and pop it into the fridge for at least 30 minutes.
Using a large cookie scoop (about 3 tablespoons of dough), drop mounds onto your lined baking sheets, leaving 2 inches of space between them. Bake for 8 minutes first.
Carefully pull the baking sheets out at the 8-minute mark. Gently press 3 to 4 of the remaining fresh mini marshmallows into the top of each warm, puffed cookie. Return to the oven for an additional 2 to 3 minutes, just until the marshmallows look beautifully puffed and lightly toasted.
