Heat oven to 350°F (325°F for dark or nonstick pan). Line 15x10x1-inch pan with foil. Spray with cooking spray.
Make batter as directed on cake mix box. Pour into pan.
Bake 17 to 23 minutes or until toothpick inserted in center comes out clean. Do not remove cake from pan. Cool completely, about 30 minutes. Place cake in freezer. Freeze until firm, about 1 hour.
Meanwhile, to make roses, unwrap fruit snacks. Brush one side of each fruit snack lightly with water. Sprinkle with sugar to coat. Repeat on other side. Cut each snack into 1/2-inch strips. Starting at one short end, roll strips around one end of toothpick, gathering it together to form a rose. Add on an additional strip, pressing to hold it together. Repeat to make 8 roses.
To assemble jam stacks, remove cake from freezer. Using foil, lift cake from pan. Using 2 3/4-inch round biscuit cutter, cut 16 rounds from the cake. On cooling rack, place 8 rounds top side down. Spread each with 1 teaspoon raspberry jam. Top with remaining cake rounds, top side up.
In microwavable bowl, microwave frosting uncovered on High about 30 seconds or just until melted; stir to blend. Let cool slightly. Spoon 2 tablespoons frosting over each cake, allowing frosting to run down sides of cake. Insert rose-topped toothpick through top of each cake to secure layer and for rose to provide decoration.