Heat oven to 375° F. Spray 15-1/2 x 10-1/2 x 1-inch metal baking pan with cooking spray. In large bowl, beat butter and sugar with electric mixer until creamy. Add eggs and vanilla; beat well. Combine flour, oats, baking powder and salt; add to butter mixture and mix well. Spread evenly into prepared pan. Bake 18 to 20 minutes or until golden brown. Remove pan to wire rack. Sprinkle chocolate chips evenly over hot cookie in pan; let stand until softened, about 1 minute. With spatula or knife, spread softened chocolate evenly over cookie. Sprinkle with candy-coated chocolate pieces; press down lightly. Cool completely; cut into bars.