Combine the cocoa, flour, and salt in a small bowl. Set aside.
Using a wooden spoon or an electric mixer on low speed, blend together brown sugar, prunes, butter, egg whites, and vanilla. Gradually add flour mixture and blend after each addition. Stir in walnuts.
Spray an 8x8" pan with non stick spray. Pour batter into pan and spread evenly. Bake for 20-22 minutes, until edges feel dry to touch but center appears fudgy. Do not overbake.
Remove from oven and let cool in pan before slicing. For maximum moistness, store squares in an airtight container.
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