1. Heat oven to 350 degree F. Line bottom and sides of two 8x8x2-inch baking pans with foil; grease foil and set aside.
2. In a heavy-bottom saucepan stir together the butter and unsweetened chocolate over low heat just until melted. Remove from heat. Using a wooden spoon, stir in the granulated sugar and the vanilla. Transfer to a bowl and let cool 5 minutes.
3. Whisk in eggs, one at a time, beating after each just until combined. Stir in flour, cocoa and salt. Spread batter evenly into prepared pans. Sprinkle hazelnuts, pecans and white chocolate over top.
4. Bake 35 minutes or until a toothpick inserted in the center comes out clean. Cool brownies in pan on a wire rack, 30 minutes. Spread top of one brownie layer with the chocolate-hazelnut spread. Carefully remove second brownie layer from pan and place on top of the chocolate-hazelnut-spread layer, nut side up. Chill for 2 hours or overnight. to serve, lift whole brownie sandwich from pan while still chilled. Cut immediately. Makes 16 brownies.