Microwave Chocolate Cake

In a bowl, combine the first five ingredients. Stir in water, oil, vinegar and vanilla until well blended. Pour into an ungreased 8-in. square microwave-safe dish. Microwave on high for 6-8 minutes, turning the dish every 2 minutes, or until a toothpick inserted near the center comes out clean. In a 1-qt microwave safe bowl, combine sugar, cornstarch and cocoa. Stir in water and salt. Microwave 1 minute more. Stir in butter and vanilla. Spoon over pieces of warm cake.