In the top of a double boiler, slowly whisk 3/4 cup water into the cornstarch. Stir in the lemon juice, salt and cayenne to taste. Place the top over simmering water and cook, stirring constantly, until slightly thickened, 2 to 3 minutes. Remove the top from the heat. Whisk in the egg yolks, one at a time, and one tablespoon of the butter. Replace over the water and cook until slighly thickened, about three minutes, stirring constantly. When ready to serve, whisk in the remaining butter; taste and correct the seasonings.