In a microwave-safe bowl, combine oats and water. Microwave, uncovered, on high for 1-2 minutes or until thickened, stirring once; set aside.
In a large bowl, cream butter and sugars until light and fluffy. Beat in eggs until well blended. Beat in vanilla and oat mixture. Combine dry ingredients; gradually add to oat mixture until combined.
Pour into a greased 11x7-in. microwave-safe dish. Shield corners with small triangles of foil. Microwave, uncovered, at 50% power for 6 minutes. Cook on high for 6 minutes or until when center is touched, cake clings to finger while area underneath is almost dry. Place on a wire rack to cool.
Combine topping ingredients in a microwave-safe dish; microwave, uncovered, on high for 4-5 minutes or until thicken and bubbly, stirring every 2 minutes. Spread over warm cake.
Editor's Note: Shielding with small pieces of foil prevents overcooking of food in the corners of a square or rectangular dish. Secure foil firmly to dish and do not allow it to touch insides of microwave.