Preheat oven to 350 degrees. Line bottom and sides of an 8-inch-square baking pan with foil, leaving a 2-inch overhang. Set aside.
In a small saucepan, bring an inch of water to a simmer over low heat. Place chocolate and butter in a medium heatproof bowl; set bowl over pan. Stir occasionally until melted. Remove from heat. Immediately whisk in sugar, egg, egg white, prune puree, vanilla, and salt until smooth.
In a medium bowl, sift together flour and cocoa. Stir into chocolate mixture just until incorporated.
Spread batter in prepared pan. Bake until top is firm (a toothpick inserted in center will come out with a few moist crumbs), about 30 minutes. Let cool completely in pan. Using foil, lift brownies from pan. Peel off foil. Using a serrated knife, cut into 16 squares.
Look for the puree in the baby-food or baking section at the grocery store.