Combine beans, herbs, salt, pepper, cayenne, cumin, chili and lemon zest in food processor. Pulse until moderately smooth. Stir in flour (do not run processor!).
Heat 3" of vegetable oil in a pot to 375°F Using 2 tablespoons, make oval dumplings (quenelles) from the mixture, and drop into hot oil. Deep fry until dark golden brown. Drain briefly on kitchen paper and sprinkle lightly with salt.
Stir together mint, yogurt, lemon juice, salt and pepper.
Serve felafel with yogurt sauce and a lightly-dressed salad.