Preheat oven to 350°F and lightly butter an 8x-inch loaf pan.

To prepare prune purée, place prunes and water in a food processor and process until puréed.

Whisk together flour, cocoa, sugar, baking soda, espresso powder and salt in a large bowl. In a separate bowl, whisk together the prune purée, oil, vinegar and vanilla. Stir the wet ingredients into the dry ingredients until a stiff 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 out onto a wire rack and cool completely.