BAKE cake according to package directions for a 9" X 13" cake and allow to cool completely. Once cool, remove from pan onto wax paper on working surface.
CUT a piece of twine the length of the longest side of the cake. Wrap twine ends around fingers of both hands and stretch out. Align twine on middle of short side of cake this is how you'll cut the cake into half of its height. Pull each end gently consecutively from left to right as you move across the cake.
Make sure to stay on center of cake so your cut is even. Using both hands or flat cookie sheet, separate cakes.
CUT out stars using cookie cutter. Remove scraps from around stars.
WHIP cream cheese, butter and vanilla in large bowl with mixer until creamy. Turn the mixer to low speed and slowly add sugar. Whip until smooth. Fill a decorating bag with a star decorating tip OR a sandwich size zip-top plastic bag with the frosting. If using plastic bag, snip a half inch opening on one of the bottom corners with kitchen shears.
FROST the top of each star. Sprinkle star sprinkles on frosting and place one blue STARMALLOW on center.