Heat oven to 350 degrees. Oil bottom of 23 cm square pan. Place egg whites in a large bowl. Beat until slightly frothy. Add oil, yogurt and vanilla, mix well. Add sugar and cocoa, mix well. Add flour and salt. Mix until blended. Pour into pan. Bake at 180ÂșC or 26-28 minutes. Avoid over baking. Dust with confectioners sugar.