In large mixing bowl, combine butter, oats, flour, brown sugar, walnuts, eggs and vanilla.
Beat 3 minutes at medium speed until light and creamy.
Stir in 1 cup chocolate chips.
Spread in microwave baking dish 20 cm.
Cover dish with waxed paper or plastic wrap.
Microwave on high power 4 minutes, rotate dish, microwave 4 minutes longer or until top of cake appears dry and cake has an even color.
Sprinkle 1 cup chocolate chips over warm cake.
Let stand 10 minutes. Spread evenly over cake.
Cool on countertop. Serves 8-12.