1 tbsp. butter
1 egg yolk
1 tsp. lemon juice
Salt
Cayenne

Melt butter in a Pyrex bowl that is placed in a skillet containing hot, not boiling, water. DO NOT USE A DOUBLE BOILER. Beat egg yolk in a saucer and add the lemon juice, salt and cayenne. Add this to the melted butter and stir until it thickens. Should it get too thick, add a little hot water from the skillet. This keeps indefinitely in the refrigerator. To reheat, just set the bowl in a little hot water and add a bit of the hot water to the sauce. Stir until smooth.