Ham and Watercress Crepes

Blend all crêpe ingredients except butter in a blender until smooth. Chill batter 30 minutes. Brush a 10-inch nonstick skillet with some butter, then heat over moderately high heat until hot but not smoking. Holding skillet off heat, pour in 1/4 cup batter, immediately swirling and tilting to create a thin, even layer. (If batter sets before skillet is coated, reduce heat slightly. Next crêpe will be better.) Return skillet to heat and cook until just set and pale golden around edges, 10 to 15 seconds. Flip crêpe carefully and cook until underside is just cooked through and pale golden, about 20 seconds more. Transfer crêpe to a large plate. Make 4 more with remaining batter (5 more if first crêpe stuck to skillet), buttering skillet each time and stacking crêpes on plate as made.