Preheat oven to 350°F. Lightly butter an 8x4 inch loaf pan. In a food processor, process prunes and water until puréed. In a bowl, whisk together flour, cocoa, sugar, baking soda, espresso powder and salt. In a separate bowl, whisk together the prune purée, oil, vinegar and vanilla. Mix the wet ingredients into the flour until a crumbly batter forms. Pour water over the top and stir completely to combine. Pour the batter into the prepared pan and bake until a cake tester inserted into the center of the loaf comes out clean, 35 to 40 minutes. Cool in the pan for 10 minutes. Turn the loaf out onto a wire rack and cool completely.