Fill a large, deep-sided skillet threefourths full with water. Bring water to barely simmering and cover. Meanwhile, in a small pot, whisk buttermilk, egg yolk, cornstarch, 1/4 tsp of the salt and the cayenne over medium heat until smooth. Cook, stirring constantly, until liquid barely simmers and is thickened, about 3 minutes, being careful not to scramble eggs. Remove from heat and stir in lemon juice, chives and butter. Cover with lid to keep warm.