Melt butter and dark chocolate together, then preheat oven to 350 °F.
Beat eggs with granulated and brown sugar until thick and pale.
Stir melted chocolate and vanilla extract into the egg–sugar mixture.
Sift in dry ingredients and gently fold in chips, nuts, and marshmallows.
Scoop dough balls, bake 8–10 minutes until edges set, then cool.
