Using the 2 tablespoons of butter, generously butter an 8-inch spring-form cake pan. Dust with zwieback crumbs and shake out excess. Beat egg whites until they are fairly stiff; but not as stiff as for meringue. Add 1 cup sugar gradually, beating until blended. Add the softened cream cheese and beat until smooth. Add 1 teaspoon vanilla and blend in, beating for 1 or 2 minutes. Pour the batter into the spring-form pan. Bake in a 350 degrees F oven for about 30 minutes, until small cracks begin to appear. Blend sour cream with remaining 2 tablespoons sugar and remaining 1/2 teaspoon vanilla, and pour on top of cheesecake. Return to oven and bake for 5 minutes at 475 degrees F. The cake is set when bubbles form at the edge. Remove from oven and let cool in pan. Refrigerate for at least 2 hours before removing sides of pan. To serve, top with your favorite fresh fruit, passing additional fruit on the side; sliced strawberries are an especially attractive topping.