Start by beating your softened butter, brown sugar, and granulated sugar together for about two minutes until the mixture looks light and creamy. Add in your eggs and vanilla extract, mixing until everything is fully combined. This step builds the base structure for your cookie dough.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Slowly add this dry mixture into your butter mixture, stirring on low speed just until the flour disappears. Overmixing at this stage can make your cookies tough instead of soft.
Once your dough comes together, gently stir in the chocolate chips by hand or on the lowest mixer setting. You want them evenly distributed throughout the dough without breaking the mixture down further.
Using a large cookie scoop, portion the dough into balls and place only six at a time on your prepared baking sheet, since these cookies spread quite a bit. Bake at 350°F for about 12 minutes, pulling them out while the centers still look a touch underdone.
While your cookies cool, place the mini marshmallows into a microwave-safe bowl and heat for 45 seconds. Stir, then continue microwaving in 15-second bursts until the marshmallows turn smooth and fully melted.
Let the melted marshmallow cool just enough to touch safely. Pinch a small amount between two fingers and gently pull apart to form thin strands, then drape them over each cooled cookie to create your web design.