In a 10-inch skillet, heat the butter and chocolate together on low until it is almost completely melted and immediately remove from heat.
Allow the residual heat to melt the remaining butter and chocolate.
Beat in the sugar until well incorporated and the mixture is ribbony.
Beat in the eggs one by one until fully incorporated.
Mix in the cocoa powder, flour, salt, and baking powder until it is fully mixed in.
Bake in the oven for 15 - 20 minutes.
Top with the marshmallows, nuts, and chocolate chips. Toast the marshmallows using a kitchen torch, or with the oven broiler. Serve warm with several scoops of vanilla ice cream