divided In large mixer bowl, mix butter, oats, flour, brown sugar, eggs, walnuts and vanilla. Beat 3 minutes at medium speed or until mixture is light and fluffy. Stir in 1 cup chocolate chips. Spread batter into 8" square microwave-safe baking dish; cover with wax paper. Cook on HIGH power 4 minutes; rotate dish 1/4 turn. Cook on HIGH power 4-5 minutes longer or until top of cakeappears dry and bottom is an even color. Sprinkle remaining 1 cup chocolate chips over top of warm cake. Let stand 10 minutes, then spread chocolate evenly over cake. Cool directly on counter top. Cut into serving pieces.