Preheat The Oven To 375F. Melt the butter with the four squares of chocolate in a large saucepan over low heat, stirring often. Remove the pan from the heat and mix in the granulated and brown sugars, eggs (one at a time), flour, salt, vanilla and nuts. Spread the batter into a well-buttered 9-by-9-by-2-inch baking pan, pushing it well into the corners and smoothing the top. Bake the brownies uncovered for 35 to 40 minutes, or until the brownies feel fairly firm and begin to pull away from the sides of the pan. Cool the brownies in the upright pan on a wire rack for at least 30 minutes. Cut them into large squares.