Coffee fiends: Go with 1 Tbsp. espresso powder. Be advised—these are sticky! Avoid them if you wear braces.
Preheat oven to 350°. Line bottom and sides of an 8-inch pan with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil.
Microwave chocolate squares and butter in a large microwave-safe bowl at HIGH 1 1/2 to 2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in granulated and brown sugars. Add eggs, 1 at a time, whisking just until blended after each addition. Whisk in flour, vanilla, and salt. Stir miniature marshmallows, caramel bits, and instant espresso into batter.
Pour mixture into prepared pan.
Bake at 350° for 44 to 46 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Cool completely on a wire rack (about 1 hour). Lift brownies from pan, using foil sides as handles. Gently remove foil, and cut brownies into 16 squares.
*12 caramels, quartered, may be substituted.
Note: Be sure to insert wooden pick into brownie, not marshmallow, when testing for doneness. (Marshmallows will rise to the top when baking.) We tested with Kraft Caramel Bits.