Heat oven to 350ºF (325º for dark or non-stick pans). Spray the bottoms of two 9-inch round cake pans.
In a large bowl, mix the cake ingredients with an electric mixer on low until the cake mix is all wet, then on high for 2 minutes. Divide the batter equally among the pans. Bake for 25-30 minutes, until a toothpick in the center comes out clean.
While the cakes are baking, beat the cream cheese and butter in a bowl on low with an electric mixer until smooth. Slowly beat in the powdered sugar 1 cup at a time until smooth. Cover and refrigerate for about 30 minutes, until it's the right consistency for spreading.
When the cakes are cool and the frosting is chilled, level the cakes and stack them with a thin layer of frosting between them. Spread a thin layer of frosting over the whole cake to capture any crumbs (the top of the cake WILL come up easily, so a crumb coat is important), and follow that up with a thicker layer of frosting. To store the cake, keep it covered in the refrigerator.