Step 1Mix rice and milk in a saucepan and bring to a simmer over medium-low heat, stirring frequently to keep rice from sticking to pan. As rice begins to cook, add 1 tablespoon sugar, salt and zest. When liquid is simmering, cover pan and continue cooking, stirring occasionally, until rice has absorbed almost all liquid and is reduced to a soft pudding, 20 minutes. Stir in butter and set aside to cool.
Step 2When rice mixture cools, add egg yolks, vanilla and flour, beating with wooden spoon after each addition.
Step 3Heat oil in large saucepan to 360 to 375 degrees. Beat egg whites with dash of salt until soft peaks form, 3 to 4 minutes. Sprinkle remaining 1 tablespoon sugar over them and continue beating until stiff. Gently fold beaten egg whites into rice mixture. (May be covered with plastic wrap and refrigerated until ready to fry.)
Step 4Working in batches, drop batter by tablespoons into hot oil and fry, turning once, until frittelle are crisp and brown, 6 minutes. Remove with a slotted spoon and roll in sugar.