Make the prune purée: Add the pitted prunes and hot water to the blender. Pulse to combine, then blend until a paste consistency forms. If needed, use a soft spatula to scrape the sides of the blender. (Purée will store well in an airtight container in the fridge for up to four weeks.)Make the brownies: Preheat the oven to 325ºF.  Spray an 8- X 8-inch baking pan with cooking spray and line with parchment paper, leaving the paper overhanging on 2 sides; spray paper with cooking spray.  In large heatproof bowl, melt chocolate with oil over simmering water. Whisk in sugar and prune purée until mixed; whisk in eggs and vanilla. Fold in flour and salt just until blended; fold in walnuts. Scrape batter into prepared pan, spreading it evenly. Bake about 30 minutes or until a cake tester inserted into the brownies comes out clean. Cool in pan at least 10 minutes; remove from pan and cut into 2-inch squares.Recipe courtesy of California Prunes.