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Preheat oven to 300°F Grease an 8x8 pan with butter.
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Put chocolate and butter in top of double boiler, stirring occasionally, until melted. Set aside off heat.
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Beat sugar and eggs with stand or hand mixer until a ribbon forms when a spoonful of liquid is allowed to fall back into the bowl.
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Add spoonfuls of warm melted chocolate, beating well after each addition until fully incorporated. Do not add too quickly, to avoid scrambling the eggs.
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Sift flour, cocoa and baking powder together. Add walnuts and toss until coated.
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Add dry ingredients to wet, and stir well until well moistened.
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Pour batter into prepared pan. Place pan in oven. Leave oven door slightly ajar for first 2 minutes, then close door and bake 20 minutes.