Preheat oven to 325 degrees F. Spray a 9-inch springform pan with nonstick spray and line the bottom with a round of parchment paper. In a large bowl, use an electric mixer to mix the cream cheese- blend until it is nice and smooth and creamy. Mix in sugar and salt and blend for 2 minutes, scraping down sides of the bowl as needed. Add eggs, one at a time, blending after each addition. Finally, mix in sour cream, whipping cream and vanilla. Mix until smooth. Pour the batter into the prepared pan. Bake the cheesecake for 45 minutes. It should be set to the touch and not jiggly. Remove the cheesecake from the oven and let it cool on a wire rack for at least an hour. When it has cooled, place the pan into the freezer and let the cheesecake freeze completely. This can be done in several hours- or overnight.
Prepare the cake layers:
Preheat oven to 325°F. Grease and flour two 9-inch round pans. It’s easiest to get the cake out of the pans if you line them with rounds of parchment paper too. In a large bowl, whisk together the cake mix, flour, sugar and salt. Add the remaining ingredients and beat with hand mixer for 2 minutes, or until well blended. Divide the batter evenly between the prepared pans. Be sure to fill them 3/4 to 1-inch below the top of the pan. Bake 35 to 45 minutes, or until a toothpick inserted into the center comes out clean. Watch closely- check after 35 minutes and then every 5 minutes until they test done.
Prepare the frosting: In a large bowl, using an electric mixer on medium-high speed, beat powdered sugar, cream cheese, butter and vanilla until it is smooth and creamy (do not over beat).
Assemble the cake: Place one cake layer into the center of a cake plate or platter. Remove the cheesecake from the freezer, take off the sides of the pan, and slide a knife under the parchment to remove the cheesecake from the pan. Peel off the parchment. Measure your cheesecake layer against the cake layers. If the cheesecake layer turns out to be a slightly larger round than your cake, move it to a cutting board and gently shave off some of the exterior of the cheesecake to get it to the same size as your cake layers. Place the cheesecake layer on top of the first cake layer. Place the 2nd cake layer on top of the cheesecake.
Frost the cake:
Use a long, thin spatula to cover the cake completely with a thin and even layer of frosting. Once the first layer of frosting is applied, add more to cover bare spots. Start by adding a large scoop of frosting onto the top of the cake. Use a long, thin spatula to spread the frosting evenly across the top and then spread it down the sides of the cake too. You can use a piping bag to add a decorative edge. Add more rainbow sprinkles on top, if desired. Keep this cake refrigerated until ready to serve. Keep leftovers refrigerated too.