Combine flour, sugar, baking powder, cinnamon and lemon rind in a bowl and make a well in the centre. Add buttermilk and egg yolks, stir until smooth, stir in apple. Set aside to rest at room temperature for 30 minutes.
Meanwhile, for cinnamon sugar, combine ingredients in a bowl, then spread on a tray and set aside.
Preheat oil in a deep-fryer or deep-sided saucepan to 180C. Whisk eggwhites and a pinch of salt in a bowl until firm peaks form. Fold one-third of eggwhite into apple mixture to lighten, then fold in remaining eggwhite. Spoon rough quenelles of mixture into hot oil, in batches, and cook, turning occasionally, until golden and cooked through (2-3 minutes; be careful as hot oil may spit). Drain with a metal sieve, toss in cinnamon sugar and serve hot with maple syrup.