Preheat The Oven To 375~. 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 9x9" baking pan, pushing it well into the corners and smoothing the top. Bake the brownies uncovered for 35-40 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.