Preheat oven to 350°F. Lightly spray 2 (9-inch) cake pans with baker’s spray.
To make the cake whisk together, in a large mixing bowl, the buttermilk, vegetable oil and eggs until fully combined. Add the dry ingredient packet from the box of red velvet cake mix. Whisk vigorously by hand, or with a handheld mixer on low, for 2 minutes or until no lumps remain.
Divide the batter evenly between the two prepared cake pans and bake for 25 to 30 minutes. A toothpick inserted into the center of your cake should come out clean. Cool in the pans for 10 minutes before turning them out onto a cooling rack to cool completely.
While your red velvet cake is baking you can prepare the cheesecake batter.
Lightly spray a 9-inch springform pan with baker’s spray and line the bottom of the springform pan with a round piece of parchment paper. In an oven-safe 9x13 pan add enough water to fill the pan about 1 to 2 inches. Set both pans aside.
In a large mixing bowl, with a handheld mixer on low, beat together the cream cheese and granulated sugar for 1 minute. Make sure that your cream cheese is smooth and without lumps.
Add the eggs and beat for another 30 seconds, on low, to completely incorporate the eggs. Add the sour cream, all-purpose flour, and vanilla extract and beat just until combined. Do not overmix the cheesecake mixture. You do not want to incorporate a lot of air into your batter.
Pour the cheesecake batter into the prepared springform pan. Add the springform pan to the middle rack in the oven and place the water filled pan on the bottom rack in the oven. This will create a moist environment for the cheesecake to cook evenly. Bake for 45 minutes or until the center is just barely jiggly.
Turn off the oven and crack the oven door open about 1 inch. Leave the cheesecake in the oven for an additional 1 hour. This allows the cheesecake to continue to cook and set as it cools. After one hour, remove the cheesecake from the oven and allow it to cool for an additional 1 hour on the counter. Don’t forget to remove the water-filled dish as well.
Once your cheesecake has cooled on the counter for 1 hour you can assemble your cake.
Place your first layer of red velvet cake onto a serving platter. Gently place the cheesecake layer, upside down, onto the bottom red velvet cake layer. Peel the parchment paper off the bottom of the cheesecake before placing the second red velvet cake layer on top of the cheesecake.
Using a bread knife, gently level off the domed part of the top red velvet cake layer. Save those scraps of red velvet cake because you can crumble them and use them as a garnish on the top of your red velvet cheesecake.
Lightly cover the cake with plastic wrap and chill in the refrigerator for 8 hours up to overnight.
Once your red velvet cheesecake has chilled, remove the plastic wrap and frost your cake with the cream cheese frosting.
To make the cream cheese frosting, you will place the unsalted butter and cream cheese into a large mixing bowl. Using a handheld mixer, on medium speed, beat the butter and cream cheese for 2 minutes or until very smooth.
Slowly add the powdered sugar, one cup at a time, until all the powdered sugar is incorporated. Add the heavy cream, vanilla extract and salt and continue to beat just until the frosting is light and fluffy. It should be stiff enough to hold its shape when frosting the cake. If the frosting is too loose, then you can add additional powdered sugar, ¼ cup at a time, to thicken it up. If it is too stiff you can add an additional 1 tablespoon heavy cream.
Reserve about a cup of the frosting and place into a small piping bag, fitted with a star tip, to add a decorative border on the top of your cake once you have frosted it. You can also add the little red velvet crumbles (from leveling off the cake layers) on the inside border of the top of your cake.