Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Prepare the cake according to the directions on the package.
In a medium bowl, combine ricotta, sugar, eggs and vanilla. Mix until smooth. Drop by spoonfuls on top of uncooked cake.
Bake at 350 degrees F (175 degrees C) for 1 1/2 hours, or until a toothpick inserted into the cake comes out clean. Sprinkle cooled cake with confectioners sugar. Keep refrigerated.
