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Preheat oven to 350 degrees F. Grease a 9x13-inch pan and set aside.
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In a heavy saucepan, combine 3/4 cup brown sugar and 1/4 cup butter. Cook and stir over low heat until butter is melted and mixture is combined. Remove from heat and stir in 1 egg, 2 tablespoons flour and 1 teaspoon vanilla and mix well. Stir in all of the nuts and set aside.
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In another heavy saucepan, combine 2/3 cup butter with cocoa. Cook over low heat, stirring frequently, until butter melts. Add sugar and 1 cup brown sugar and beat well. Then add 4 eggs and 2 teaspoons vanilla and beat to combine. Stir in 1-1/4 cups flour and baking powder.
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Pour chocolate mixture into prepared pan. Spoon the nut topping over chocolate mixture in an even layer. Bake in preheated oven for 30-40 minutes or until just barely set. Do not overbake or the topping will get too hard. Cool on a wire rack, then cut into bars.