Preheat oven to 350F. Butter the bottom and sides of a 8 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In a small, heatproof bowl, melt butter and chocolate together. Stir until combined and very smooth. Set aside to cool for 5 minutes. Whisk in sugar and salt. Add eggs one at a time, beating vigorously to blend after each addition. Stir in vanilla extract, then flour.
Spread into prepared baking pan and sprinkle walnuts evenly on top. Bake for 18-20 minutes or until a toothpick inserted near the center comes out clean (with a few moist crumbs attached). Remove pan from oven and turn off heat. Sprinkle white chocolate chips and marshmallows on top, return pan to oven for 2 minutes, then transfer pan to a rack to cool.
Once the brownies are cool, in a small bowl combine caramels and cream. Microwave on medium (50 percent) power for 1 1/2 minutes; stir until completely smooth. Drizzle caramel-cream over brownies, then let them cool completely.