Heat oven to 350°F. Generously grease 13x9-inch pan. In large bowl, combine cake mix, butter, milk and eggs; beat at low speed until combined. Beat 2 minutes at medium speed. Fold in gumdrops. Spread in greased pan.
Bake at 350°F. for 30 to 40 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Run knife around sides of pan to loosen. Cool 1 hour or until completely cooled.
In small saucepan, heat frosting over low heat just until melted. Spoon frosting over cooled bars. Sprinkle with decorator sugar. Cut into bars.