1. Prepare Peppermint Cheesecake Layers: Preheat oven to 325°. Line bottom and sides of 2 (8-inch) round cake pans with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil. Beat cream cheese, 1/2 cup sugar, and 2 Tbsp. butter at medium speed with an electric mixer 1 to 2 minutes or until creamy and smooth. Add 3 eggs, l at a time, beating until blended after each addition. Add flour and next 3 ingredients, beating until blended. Fold in candies. Pour batter into prepared pans. Place cake pans in a large pan; add water to pan to depth of 1 inch.
2. Bake at 325° for 25 minutes or until set. Remove from oven to wire racks; cool completely in pans (about 1 hour). Cover cheesecakes (do not remove from pans), and freeze 4 to 6 hours or until frozen solid. Lift frozen cheesecakes from pans, using foil sides as handles. Gently remove foil from cheesecakes. Wrap in plastic wrap, and return to freezer until ready to assemble cake.
3. Prepare Sour Cream Cake Layers: Preheat oven to 350°. Beat cake mix, next 3 ingredients, and 1/2 cup water at low speed with an electric mixer 30 seconds or just until moistened; beat at medium speed 2 minutes. Spoon batter into 3 greased and floured 8-inch round cake pans.
4. Bake at 350° for 15 to 20 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, and cool completely (about 1 hour).
5. Prepare White Chocolate Mousse Frosting: Cook 2/3 cup sugar and 1/4 cup water in a small saucepan over medium-low heat, stirring often, 3 to 4 minutes or until sugar is dissolved. Add morsels; cook, stirring constantly, 2 to 3 minutes or until chocolate is melted and smooth. Remove from heat. Cool to room temperature (about 30 minutes), whisking occasionally.
6. Beat cream and 2 tsp. vanilla at high speed with an electric mixer 1 to 2 minutes or until soft peaks form. Gradually fold white chocolate mixture into whipped cream mixture, folding until mixture reaches spreading consistency.
7. Assemble Cake: Place 1 cake layer on a cake stand or plate. Top with 1 frozen cheesecake layer. Top with second cake layer and remaining cheesecake layer. Top with remaining cake layer. Spread top and sides of cake with frosting. Chill until ready to serve. Garnish, if desired.