Preheat oven to 350 degrees F (175 degrees C). Lightly grease an 8x8 inch baking pan.
Melt chocolate in a large heat proof bowl over a pan of simmering water or in a microwave oven. Stir frequently until smooth then stir in the boiling water. Remove from heat and set aside to cool slightly.
Whisk the egg whites and vanilla into the chocolate mixture. Combine the sugar, flour, baking powder and salt; stir into the chocolate mixture just until blended. Spread evenly into the prepared pan.
Bake for 20 to 25 minutes in the preheated oven, or until the edges pull away from the sides of the pan. Let cool completely before cutting into squares and dusting with confectioners' sugar.