1. Preheat oven to 350 degrees and prepare a 9 or 10 inch cake pan by placing parchment paper in the bottom of the pan and spraying the sides with a non stick spray.
2. Cream butter and sugars together until light and fluffy, about 3-4 minutes.
3. Mix in egg and vanilla extract.
4. Add flour, cocoa and baking soda and mix until combined.
5. Stir in chocolate chunks and nuts. Dough will be thick.
6. Spread half of the dough evenly into the cake pan.
7. Spread marshmallow fluff on top of cookie dough.
8. Drop remaining cookie dough over the marshmallow fluff and flatten into an even layer.
9. Bake for 18-20 minutes. The center may still look a little undercooked but it will firm up and continue cooking as it cools.
10. Remove from oven and let cool completely in pan, then carefully remove.
11. When ready to serve, top cookie with marshmallows, chocolate chips and nuts and bake on convection bake for about 15 minutes, until marshmallows are lightly toasted.
12. Allow to cool for about 10 minutes, then serve.