PREHEAT the oven to 350° F. Spray 8-inch-square baking pan or dish with nonstick cooking spray.
COMBINE flour, 3/4 cup sugar, 3 tablespoons cocoa, baking powder and salt in medium bowl. Add 1/2 cup evaporated milk, oil and vanilla extract; whisk until just blended. Spread batter into prepared baking pan.
COMBINE remaining 1/2 cup sugar and 1/4 cup cocoa in small bowl. Microwave remaining 1 cup evaporated milk and water in small, uncovered microwave-safe bowl on HIGH (100%) power for 1 minute. Whisk sugar-cocoa mixture into milk mixture until blended. Gently pour over chocolate batter in pan.
BAKE for 20 to 25 minutes (25 to 30 minutes if using glass dish) or until cake layer forms on top and edges are bubbly. Let stand for 10 minutes. Spoon into serving dishes, spooning chocolate sauce over cake. Top with whipped topping.
• Individual servings can be reheated in microwave for 10 seconds.