In a saucepan, bring a cup of water to a simmer over low heat. Place chocolate and butter in a bowl and set bowl over pan. Stir occasionally until melted.
Remove and keep aside and immediately whisk in sugar, egg white, prune puree, vanilla, and salt until smooth.
In another bowl, sift together flour and cocoa powder. Stir into chocolate mixture until mixed well.
Spread the batter in a greased pan. Bake until top is firm, or for about 30 minutes.
Let it cool completely in the pan. Cut the guilt-free brownie into equal squares and enjoy!