Preheat the oven to 350º. Spray a 9 x 9-inch baking dish with nonstick spray.
Place the cake mix in a large bowl. Whisk together the buttermilk, 1 egg and oil in a small bowl until blended. Add the buttermilk mixture to the cake mix, stirring just until blended. Pour the mixture into the baking dish.
Beat the cream cheese until smooth. Beat in the sugar, yogurt, 1 egg, vanilla and butter. Spread over the cake mixture. Bake 40 – 50 minutes. You want the center to be a little gooey, so do not over bake.