Microwave chocolate squares in a small microwave-safe bowl at MEDIUM (50% power) for 30-second intervals until melted (about 1 1/2 minutes total time). Stir chocolate until smooth.
Beat 1 cup butter and 2 cups sugar at medium speed with an electric mixer until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition. Add melted chocolate, beating just until blended.
Add flour and salt, beating at low speed just until blended. Stir in vanilla, 1/2 cup pecans, and 1/2 cup chocolate morsels. Spread batter into a greased and floured 13- x 9-inch baking pan. Sprinkle with remaining 1/4 cup pecans and 1/4 cup chocolate morsels.
Bake at 350° for 40 minutes or until set. Cool completely on a wire rack.
Melt 3/4 cup butter in a large saucepan over low heat. Stir in 2 cups sugar, buttermilk, marshmallows, corn syrup, and baking soda. Cook over medium heat, stirring occasionally, 20 to 25 minutes or until a candy thermometer registers 234° (soft-ball stage). Remove from heat, and pour mixture immediately into a large mixing bowl. Beat mixture with an electric mixer at high speed 5 minutes or until mixture thickens and begins to lose its gloss. Spread Caramel Frosting evenly over brownies.
[B "Note:"] Freeze brownies in an airtight container up to 1 month.