Preheat oven to 350º and butter and flour two 9" cake pans. Prepare red velvet cake batter according to box instructions.

Divide batter among prepared cake pans and bake until a toothpick inserted in the center comes out clean, 30 to 32 minutes. Let cool slightly in cake pans, then invert onto a wire rack to cool completely. Using a serrated knife, level out tops of the cake so they're flat.

Make cheesecake layer: Line a 9" springform pan with parchment and spray with cooking spray. In a large bowl, beat cream cheese and sugar until fluffy and combined, 3 minutes. Add eggs, one at a time, until combined. Add sour cream, flour, vanilla, and salt and beat until combined.

Pour filling into prepared springform pan and bake until only slightly jiggly in the center, about 1 hour. Let cool slightly, then transfer to the freezer to cool completely and firm up before assembling.

Make cream cheese frosting: In a large bowl, beat cream cheese and butter until light and fluffy, then add powdered sugar and vanilla and beat until combined.

Assemble cake: On a cake platter, place one red velvet cake. Top with cheesecake layer, pressing down firmly, then top with second red velvet cake.

Using an offset spatula, frost cake with cream cheese frosting.

Garnish with red velvet cake crumbs before serving.