In large heatproof bowl set over hot water, melt chocolate with water, stirring until smooth. Remove from heat; let cool slightly.§ whisk in egg whites and vanilla. Stir together Sugar, flour, baking powder and salt; stir into chocolate batter just until combined. Pour into lightly greased 8 in. square cake pan. Bake in 350f oven for 20 to 25 minutes or until edges pull away from pan. Let cool on rack; dust with icing sugar.