Preheat oven to 325°F. Line a springform pan for the cheesecake.
Beat cream cheese, sugar, vanilla, and eggs until smooth. Add sour cream.
Bake cheesecake in a water bath for 40–45 min. Cool gradually, then refrigerate 4+ hrs.
Prepare cake batter: melt white chocolate and butter, then combine with sugar, eggs, buttermilk, and dry ingredients.
Divide batter into two pans and bake at 350°F for 25–30 minutes.
Cool cakes completely. Level if needed.
Stack: cake layer, cheesecake layer, top cake layer.
Frost with cream cheese frosting or white chocolate ganache.
Chill before slicing for best results.
