1. Melt chocolate base: In a heatproof bowl over simmering water (double boiler), melt chocolate and butter until smooth. Remove from heat. Stir in marshmallows until fully melted and glossy. Cool 5–10 minutes (still warm but not hot).
2. Whip egg whites: In a clean bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar; beat until stiff, glossy peaks form. Mix in vanilla.
3. Fold together: Gently fold ⅓ of whipped egg whites into chocolate to lighten. Then fold in remaining whites until no streaks remain.
4. Chill: Spoon into serving glasses or bowls. Refrigerate at least 2 hours, or until set.
5. Serve cold, topped with whipped cream, chocolate shavings, or a sprinkle of graham cracker crumbs.

💡 Pro Tips:
– For egg-free version: Fold melted chocolate-marshmallow mix into 1 (8 oz) tub whipped topping instead.
– Use good-quality chocolate—it makes all the difference!
– Make ahead: Keeps well covered in fridge up to 2 days.

Prep Time & Nutrition (per serving, serves 4):