PREHEAT oven to 375° F.
BEAT butter, brown sugar and granulated sugar in large bowl on medium speed until creamy. Add egg and vanilla extract; beat well. Add combined flour, baking soda and salt; beat on low speed just until blended. Stir in oats and 1 cup chunks. Spread dough evenly onto bottom of ungreased 13 x 9-inch baking pan.
BAKE for 25 to 30 minutes or until light golden brown. Remove from oven. Carefully sprinkle remaining chunks evenly over hot cookie in pan. Let stand 2 to 3 minutes. Spread softened chocolate evenly with spatula. Cool completely in pan on wire rack. Cut into bars. Store tightly covered at room temperature.