Notes: If you need to hold sauce up to 30 minutes, nest pan in hot tap water and stir often; add more hot water to pan to keep warm.
Prep and cook time: about 10 minutes
In a 1- to 2-quart pan over low heat, whisk to blend egg yolks and lemon juice. Add butter or margarine, in chunks, and whisk until butter melts and sauce thickens. Use at once.