Since the eggs aren't cooked all the way through be sure to use pasteurized eggs for this recipe.
Place the prunes in a heat-proof bowl. Pour the boiling water over and let sit until cooled slightly and softened, 10 to 20 minutes. Transfer to a blender and puree until smooth, about 40 seconds.
Meanwhile, whisk together the all-purpose flour, whole-wheat flour, cocoa, baking powder and salt in a medium bowl.
Beat the sugar, butter and melted chocolate in a large bowl with an electric mixer. Beat in the eggs until light, about 40 seconds. Beat in the oil and vanilla and then the pureed prunes. With a rubber spatula stir the flour mixture.
Coat the insert of a slow cooker with cooking spray. Pour in the cake batter. Cover slow cooker, and cook for two hours on the low setting. Unplug the slow cooker. Let the cake sit in the slow cooker, covered, for 30 minutes before serving. Serve sprinkled with powdered sugar, if desired.