Butter two loaf pans. You can use two 9×5 loaf pans.
Whip up cake mix according to package directions.
Divide among your loaf pans.
Beat cream cheese and sour cream in a stand mixer until smooth.
Beat in egg, sugar, and vanilla and mix until smooth.
Drop spoonfuls of the cheesecake batter randomly over the top of the loaves. Using a butter knife or the back of a wooden spoon, swirl the cheesecake into the loaves. Be careful not to swirl too much or they will just blend.
Bake for 30-35 minutes. Cool completely and then remove from pans.
To make the frosting
Mix butter and cream cheese until smooth with a hand mixer.
Mix in powdered sugar, 1 cup at a time.
Add vanilla and 1 teaspoon heavy cream and mix until smooth.
Add more cream, 1 teaspoon at a time, until the frosting reaches desired consistency. Store in refrigerator.