1. Heat oven to 450 degrees F.
2. Beat ricotta, eggs, sugar and rind in bowl until smooth, 3 to 5 minutes.Topping:
3. Heat 1/2 cup sugar in small saucepan over medium-low heat until golden and sugar dissolves, 5 minutes.
4. Pour topping into 10-inch round cake pan; swirl so sugar coats bottom and 1/2 inch up sides of pan. Pour cheese mixture into pan. Set pan inside large roasting pan. Place in oven. Fill roasting pan with hot water to depth of 1 inch.
5. Bake in 450 degree F oven for 10 minutes. Lower heat to 350 degrees F. Bake 35 minutes. Top of cake will turn golden yellow with ring of golden brown toward edges. Turn off oven; leave cake in oven with door closed for 1 hour. Cake will appear loose at first but will firm up as it sits.
6. Remove cake from oven; remove from water. Cover pan with aluminum foil. Refrigerate at least 12 hours.
7. To unmold, run sharp knife around inside edge of pan. Place bottom of pan in warm water for about 15 seconds. Wipe pan dry. Cover with rimmed plate. Invert, tap pan top gently. Pull pan off cake. Garnish with kumquats, if desired. Serve chilled or at room temperature. Store in refrigerator. Makes 12 servings.