Heat oven to 350°F. Line bottom of 13x9-inch baking pan with waxed paper; spray with no-stick cooking spray. Set aside.
Place all cake ingredients except sprinkles in bowl; beat as directed on package. Stir in orange sprinkles.
Pour batter into prepared cake pan. Bake 29-32 minutes or until toothpick inserted in center comes out clean. Cool 1 hour. Cover with aluminum foil; freeze in pan 2 hours.
Combine powdered sugar and 3/4 cup butter in bowl; beat at medium speed until well mixed. Continue beating, adding enough half & half for desired frosting consistency.
Remove cake from freezer; invert cake onto large cutting board. Carefully peel off waxed paper. (If cake is not releasing out of pan, place warm, damp dish towel over inverted pan to help release cake.)
Cut cake according to diagram using ruler or straight edge to guide knife.
Place 2 small triangles together, top-side down, on large rectangle platter or baking sheet covered with aluminum foil, to form 1 large triangle. Frost top of triangle. Carefully place large triangle on top of frosted triangle. Trim all edges, if necessary, to even them out. Working quickly, frost cake.
Use knife tip or toothpick to draw lines for separating decorator sugars. To keep decorator sugar colors from blending, cover sections that are not being sprinkled with waxed paper. Generously sprinkle wide end of triangle with yellow sugar, middle section with orange sugar and pointed section with white sugar. Decorate sides of cake with candy corn, if desired.