Preheat the oven to 350 degrees F.
Coat an 8-inch square baking pan with non-stick spray and set aside.
In a glass mixing bowl, melt the chocolate and butter together in the microwave on high for 1 to 2 minutes, or until the butter is just melted. Mix it up to melt the chocolate completely. Stir the sugar into the melted chocolate until dissolved. Mix in the vanilla and eggs. Stir in the flour and salt just until combined. Spread the batter evenly into the prepared pan. Bake for 25 to 30 minutes, or until wooden toothpick inserted in the center comes out clean.
Immediately sprinkle the marshmallows and nuts evenly over the brownies. Stick it under the broiler for 2 minutes, or until the marshmallows are light golden brown. Allow to cool to room temperature. Drizzle the caramel or chocolate sauce over the cooled brownies before cutting.