Preheat oven to 350°. Line an 8-inch square metal baking pan with a 12-inch sheet of foil, then grease foil.
Melt butter and chocolate in a medium saucepan over medium-low heat, whisking lightly until smooth and glossy. Remove from heat.
Whisk in sugar and salt, then beat in eggs one at a time until well blended. Stir in vanilla extract, then flour. Scrape into prepared pan and sprinkle walnuts evenly on top.
Bake brownies until set and slightly springy to touch, about 20 minutes. Remove pan from oven and turn off heat. Sprinkle chocolate chips and marshmallows on top, return pan to oven for 2 minutes, then transfer pan to a rack to cool.
Combine caramels and cream in a small bowl and microwave on medium (50 percent) power for 1 1/2 minutes; stir with a fork until completely smooth. Drizzle caramel-cream over brownies, then let them cool completely.
When cool, remove brownies from tray by lifting out foil, then transfer them to a board. Cut brownies into four pieces, then cut each quarter into four individual pieces.