1. Preheat oven to 350°. Prepare cake batter as directed. Pour batter into a greased and floured 13- x 9-inch pan.
2. Bake at 350° for 28 to 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes. Remove from pan to wire rack, and cool completely (about 1 hour).
3. Invert cake onto a cutting board. Cut off rounded top of cake. Trim and discard edges of cake.
4. Cut cake in half crosswise. Cut each piece in half horizontally. Carefully lift top piece off of each cake half.
5. Stir jam in a small bowl until smooth and spreadable. Spread jam over bottom halves of cakes. Replace top halves of cakes. Cut each cake half into 12 squares. Transfer petits fours to a serving platter or cake pedestal.
6. Remove lid and foil from frosting. Microwave frosting at HIGH 20 seconds, stirring after 10 seconds. Stir until smooth and spreadable. Transfer frosting to a zip-top plastic freezer bag. Snip 1 corner of bag to make a small hole. Pipe frosting onto top of petits fours, allowing frosting to drip down sides.