1. Prepare Chiffon Cake Layers: Preheat oven to 325°. Lightly grease bottoms of 4 disposable cake pans; line bottoms with wax paper, and lightly grease.
2. Whisk together flour, next 2 ingredients, and 1 cup sugar in bowl of a heavy-duty electric stand mixer.
3. Whisk together oil and next 3 ingredients; add to flour mixture, and beat at medium speed 1 to 2 minutes or until smooth.
4. Beat egg whites at medium speed until foamy. Add cream of tartar; beat at high speed until soft peaks form. Gradually add remaining 1/4 cup sugar, 1 Tbsp. at a time, beating until stiff peaks form and sugar dissolves. Gently stir one-fourth egg white mixture into flour mixture; gently fold in remaining egg white mixture. Divide batter among prepared pans, spreading with an offset spatula. Sharply tap pans once on counter to remove air bubbles.
5. Bake at 325° for 14 to 16 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes. Remove from pans to wire racks; discard wax paper. Cool completely. Wrap layers in plastic wrap, and freeze 12 hours.
6. Meanwhile, prepare Ice-Cream Layers: Line 3 disposable cake pans with plastic wrap, allowing 6 to 8 inches to extend over sides. Stir together ice cream and candies in a large bowl. Divide mixture among prepared pans (about 2 1/2 cups per pan), spreading to within 1/2 inch of sides of pans. (The cake layers shrink a little as they cool, so this helps ensure that the ice-cream layers will be the same size.) Cover with plastic wrap, and freeze 12 to 24 hours.
7. Assemble Cake: Remove plastic wrap. Place 1 layer of cake on a serving plate; top with 1 layer of ice cream. Repeat with remaining layers of cake and ice cream, ending with cake on top. Wrap entire cake with plastic wrap, and freeze 12 to 24 hours.
8. Prepare Frosting: Beat cream and vanilla at medium speed until foamy. Increase speed to medium-high, and gradually add powdered sugar, beating until stiff peaks form. (Do not overbeat or cream will be grainy.) Remove cake from freezer. Spread top and sides with Whipped Cream Frosting. Serve immediately, or freeze up to 12 hours. Store in freezer.