Step 1: Get Ready! Preheat your oven to 410°F (210°C). Line a couple of baking sheets with parchment paper.
Step 2: Cream the Butter and Sugars. In a stand mixer with the paddle attachment, cream together the cold cubed butter, packed brown sugar, and granulated sugar on medium speed for about 2 minutes, until the mixture resembles wet sand.
Step 3: Add the Eggs and Vanilla. Add the cold eggs one at a time, mixing until each is fully incorporated. Mix in the vanilla extract until smooth.
Step 4: Whisk the Dry Ingredients. In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until evenly distributed.
Step 5: Combine Wet and Dry. Reduce the mixer speed to low. Gradually add the dry ingredients to the wet ingredients, mixing just until the flour disappears and the dough comes together. Do not overmix.
Step 6: Fold in the Goodies! Take the bowl off the mixer. Add the chocolate chips, mini marshmallows, and chopped nuts to the dough. Use a rubber spatula to gently fold everything in until evenly distributed.
Step 7: Shape Those Giants! Using a large cookie scoop (approx. 6 ounces/170g) or your hands, form the dough into large, tall, round balls. Do not flatten them.
Step 8: Bake! Place the large dough balls onto your prepared baking sheets, leaving plenty of space between them (4-6 cookies per sheet).
Step 9: Bake 'em Up! Bake in the preheated 410°F (210°C) oven for 9-12 minutes. The edges should look set and slightly cracked, but the centers should still look soft, puffed, and slightly underbaked.
Step 10: Cool Down. Leave the baked cookies on the hot baking sheets for at least 15 minutes to continue setting up. Carefully transfer them to a wire rack to cool completely afterwards.