Whisk together flour, baking powder, baking soda and salt in a large bowl.
Pulse sugar with zest in food processor til sugar is orange and moist. Transfer to bowl and whisk in buttermilk, butter and egg til smooth.
Add buttermilk mixture to flour mixture and stir til a dough forms. Dough will be sticky. Turn out dough onto a well floured surface. Dust dough with flour, the pat into a 9 inch round, about 1/2 inch thick, with floured hands. Cut out rounds with floured 2 1/2 inch cutter then cut out center holes with floured 1 inch cutter. Transfer doughnuts and holes to a lightly floured baking sheet. Reroll scraps once and cut out additional rounds and holes.
Heat 2 1/2 inches oil to 375 in a wide 4 to 5 quart heavy pot. Fry doughnuts and holes in separate batches of 6 or 7, turning over once, til golden brown, 45 seconds to 1 1/4 minutes total per batch. Transfer with a slotted spoon to paper towels to drain, then cool on rack til barely warm. Return oil to 375 between batches.
Stir together sugar and cinnamon in bowl and coat half of doughnuts and holes. Heat cream til hot, then stir in chocolate and corn syrup til smooth. Stir in warm water. Dip tops of remaining doughnuts and holes in chocolate and transfer to a rack to set, about 1 hour.
Note: Doughnuts can be fried, cooled and dredge in sugar or iced 4 hours ahead and kept at room temperature To make doughnuts with only 1 coating, double recipe for coating.