Preheat oven to 175°C. Grease a 20cm by 20cm baking pan. Line with foil; grease foil.
In a 2,8-litre saucepan, combine chocolates, 4 tablespoons butter and ¼ teaspoon salt. Melt on medium-low for 4 minutes or until smooth, stirring. Remove from heat; stir in ¾ cup sugar and vanilla. Stir in eggs, 1 at a time, until well blended.
Sift cornflour and cocoa into pan; fold to incorporate. Stir batter vigorously for at least 1 minute or until it thickens slightly and begins to pull away from sides of pan. Fold in 1 cup chopped nuts. Spread evenly into prepared baking pan. Bake for 25 to 27 minutes or until a toothpick inserted 5cm from the edge comes out almost clean. Cool in pan on a wire rack.
In a 2,8-litre saucepan, stir remaining sugar, water, golden syrup and salt. Cook on medium-high for 9 to 12 minutes or until sugar dissolves and mixture is amber, swirling occasionally to get an even colour. Working quickly, remove from heat and add cream. Once bubbling begins to subside, stir in remaining butter until incorporated. Stir in ½ cup chopped nuts.
Pour caramel over cooled brownie, tilting pan to spread evenly. Let it stand for at least 2 hours or until caramel has set. Cut into 2,5cm squares. Store in an airtight container, layered with wax paper, at room temperature for up to 2 days.