In 1 1/2-quart saucepan, vigorously stir egg yolks and lemon juice with wire whisk. Add 1/4 cup of the butter. Heat over very low heat, stirring constantly with wire whisk, until butter is melted.
Add remaining 1/4 cup butter. Continue stirring vigorously until butter is melted and sauce is thickened. (Be sure butter melts slowly so eggs have time to cook and thicken sauce without curdling.) If the sauce curdles (mixture begins to separate), add about 1 tablespoon boiling water and beat vigorously with wire whisk or hand beater until it's smooth.
Serve immediately. Store covered in refrigerator. To serve refrigerated sauce, reheat over very low heat and stir in a small amount of water.