In a large bowl, cream together 1 pound each of cream cheese, softened, and ricotta and 1 1/2 cups sugar and beat in 4 large eggs at room temperature, one at a time, beating well after each addition. Add 1/2 stick butter (1/4 cup), melted and cooled, 3 tablespoons each of flour, cornstarch, and 2 1/2 teaspoon vanilla and beat the mixture until it is well combined.

Fold in 2 cups sour cream, pour the batter into an ungreased 9 inch spring form pan and bake the cake in the middle of a preheated 325 degree oven for 1 hour. The cake will be soft in the center. Turn off the heat (do not open the oven door) and let the cake stand in the oven for 2 hours. Let the cake cool in the pan on a rack and chill it loosely covered for at least 2 hours. Remove sides of pan and transfer cake to a cake stand or plate.