1. Prepare Cheesecake Layers: Preheat oven to 300°. Line bottom and sides of 2 disposable cake pans with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil.
2. Microwave white chocolate morsels in a microwave-safe bowl according to package directions; cool 10 minutes.
3. Beat cream cheese and melted chocolate at medium speed with an electric mixer until creamy; gradually add 1 cup sugar, beating well. Add 2 eggs, 1 at a time, beating just until yellow disappears after each addition. Stir in 1 Tbsp. vanilla. Pour into prepared pans.
4. Bake at 300° for 30 to 35 minutes or until almost set. Turn oven off. Let cheesecakes stand in oven, with door closed, 30 minutes. Remove from oven to wire racks; cool completely (about 1 1/2 hours). Cover and chill 8 hours, or freeze 24 hours to 2 days.
5. Prepare Red Velvet Layers: Preheat oven to 350°. Beat 1 cup butter at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add 2 1/2 cups sugar, beating until light and fluffy. Add 6 eggs, 1 at a time, beating just until blended after each addition.
6. Stir together flour and next 2 ingredients; add to butter mixture alternately with sour cream, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in 2 tsp. vanilla; stir in food coloring. Spoon batter into 3 greased and floured 8-inch disposable cake pans.
7. Bake at 350° for 20 to 24 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; cool completely (about 1 hour).
8. Prepare Frosting: Whisk together chocolate and 1/2 cup boiling water until chocolate melts. Cool 20 minutes; chill 30 minutes.
9. Beat 1 cup butter and chilled chocolate mixture at low speed until blended. Beat at medium speed 1 minute. Increase speed to high; beat 2 to 3 minutes or until fluffy. Gradually add powdered sugar and salt, beating at low speed until blended. Increase speed to high; beat 1 to 2 minutes or until smooth and fluffy.
10. Assemble Cake: Place 1 layer Red Velvet on a serving platter. Top with 1 layer Cheesecake. Repeat with remaining layers of Red Velvet and Cheesecake, alternating and ending with Red Velvet on top. Spread top and sides of cake with White Chocolate Frosting. Store in refrigerator.