PREHEAT oven to 350° F. Grease bottom of 13 x 9-inch baking pan.
COMBINE oats, flour, butter and brown sugar in small bowl. With clean fingers, mix until crumbly; set aside.
COMBINE flour, oats, baking powder and salt in large bowl. Combine brown sugar and eggs with wire whisk. Whisk in 1 1/4 cups milk, melted butter and vanilla extract. Add to flour mixture all at once; add apples. Stir gently until just combined. Pour into pan. Sprinkle with 1 cup morsels; crumble topping evenly over morsels.
BAKE for 40 minutes or until golden brown and wooden pick inserted in center comes out with a few moist crumbs clinging to it. Remove from oven to wire rack.
MICROWAVE remaining 1/3 cup morsels and 1 1/2 teaspoons milk in small, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power. The morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until smooth. Carefully drizzle over hot cake in pan. Cool in pan at least 30 minutes. Cut into squares; serve warm or at room temperature with ice cream, if desired. Store tightly covered at room temperature.