Preheat oven to 375 degrees F and grease a 9x13 inch pan.
In a mixing bowl, combine flour, cocoa, baking soda, and salt.
In a standing mixer, beat butter, sugars, and vanilla until creamy. Beat in the eggs. Mix the flour in a little at a time until combined. Stir in the chocolate chips by hand.
Spread about two thirds of the cookie dough into the bottom of the pan. Dollop marshmallow creme onto dough and then spread it out as best you can (it's kinda tricky/sticky so you might want to use two spatulas). Sprinkle nuts over the creme and then dollop the remaining dough on the top and spread it slightly still allowing some of the marshmallow fluff to show through (again, use two spatulas or your hands).
Bake for 20-22 minutes. Cool and then cut into bars.