Preheat your oven to 350°F. Grease and line three 9-inch round cake pans with parchment paper.

Prepare your cake batter according to package directions. Divide the batter evenly among the three prepared pans. Bake for 25-30 minutes until a toothpick inserted in the center comes out clean. Allow cakes to cool completely in pans for 10 minutes, then transfer to wire racks.

Line a 9-inch springform pan with plastic wrap, leaving plenty of overhang. Trim the tops of the cooled cakes to create flat surfaces. Place one cake layer in the bottom of your springform pan.

Spread the softened strawberry ice cream evenly over the first cake layer, about 1/2 inch thick. Add the next layer of orange sherbet, spreading smoothly. Continue with lemon sorbet, then the second cake layer.

Repeat the process with mint chocolate chip ice cream, blueberry ice cream, and grape sorbet. Top with your final cake layer. Press down gently but firmly. Cover the entire cake tightly with plastic wrap. Freeze for at least 8 hours or overnight.

Prepare your whipped cream frosting by beating heavy cream with powdered sugar and vanilla until stiff peaks form. Remove the cake from the freezer. Release the springform pan sides. Transfer the cake to a serving platter. Frost the entire cake with whipped cream and decorate with rainbow sprinkles. Return to freezer for 30 minutes to set the frosting.