Preheat oven to 350 degrees F. Grease 8- by 8-inch baking pan. Line with foil; grease foil.
In 4-quart saucepan, combine chocolates, 4 tablespoons butter, and 1/4 teaspoon salt. Melt on medium-low 4 minutes or until smooth, stirring. Remove from heat; stir in 3/4 cup sugar and vanilla. Stir in eggs, 1 at a time, until well blended.
Into same pan, sift cornstarch and cocoa; fold to incorporate. Stir batter vigorously for at least 1 minute or until batter thickens slightly and begins to pull away from sides of pan. Fold in 1 cup chopped nuts. Spread evenly into prepared baking pan. Bake 25 to 27 minutes or until toothpick inserted 2 inches from edge comes out almost clean. Cool in pan on wire rack.
While brownie cools, in 4-quart saucepan, stir remaining 3/4 cup sugar, water, corn syrup, and 1/4 teaspoon salt. Cook on medium-high 9 to 12 minutes or until sugar dissolves and mixture is amber, swirling occasionally to get even color. Working quickly, remove from heat and add cream. Once bubbling begins to subside, stir in remaining 2 tablespoons butter until incorporated. Stir in remaining chopped nuts.
Pour caramel over cooled brownie, tilting pan to spread evenly. Let stand at least 2 hours or until caramel has set. Cut into 1-inch squares. Store brownies in airtight container, layered with waxed paper, at room temperature up to 2 days.
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