In medium-size nonstick saucepan, combine water, butter, cocoa powder and espresso powder, if using. Bring mixture just to a boil, stirring constantly, over medium-high heat, about 2 minutes. Gradually stir in flour mixture until blended. Reduce heat to medium. In small bowl, whisk together buttermilk, egg and vanilla. Add to cocoa mixture in saucepan; stir vigorously for 1 minute or until well blended. Pour batter into prepared pan.