Preheat oven to 350 degrees. Line an 8-in. square pan with parchment with overhang all around. Grease bottom of pan. Grind cacao nibs and sift to obtain 1/4 cup of grounds. Melt margarine in small saucepan. In mixing bowl combine sugars, ground nibs and salt. Remove margarine from heat and blend in small broken pieces of chocolate until they are melted and mixture is glossy and smooth. Add chocolate mixture to sugar mixture and blend. Beat in eggs. Mix in flour until just combined. Stir in walnuts. Pour batter into pan. Bake until set and a toothpick inserted into the center comes out with moist crumbs, about 40 min. Let cool slightly in pan, about 15 min. Lift brownies from pan using parchment. Remove parchment with brownies, and transfer to a wire rack. Let cool completely. Cut and eat.
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