In the top of a double boiler, or in a bowl set over a pan of barely simmering water, whisk the egg yolks with the lemon juice and 2 teaspoons water until egg yolks are thick and pale yellow. Remove the double boiler or bowl and saucepan from the heat and gradually add the butter, whisking constantly to thicken. Add enough tepid water to thin to pouring consistency. Add the tasso, salt, and cayenne and whisk well to blend. Adjust the seasoning, to taste.
Serve immediately, or cover to keep warm for up to 10 minutes, whisking occasionally to keep from separating.
*Tepid water is warm water, specifically 2 parts cold water to 1 part boiling water.