Preheat oven to 350°F (175°C).
Grease your baking dish or line with parchment.
Melt butter in a saucepan on medium heat.
Remove from heat and stir in cocoa powder.
Mix in sugar until fully combined.
Add eggs one at a time, mixing well.
Stir in vanilla extract gently.
Sift in flour and salt.
Fold in chocolate chips and nuts.
Pour batter into prepared baking dish.
Bake for 25–30 minutes until center is set.
Remove and top evenly with marshmallows.
Return to oven for 3–4 minutes only.
Let marshmallows puff and slightly brown.
Remove and cool completely on a rack.
Slice into squares and serve warm.