Hollandaise is a perfect accompaniment to fish, eggs benedict, asparagus, etc. 

Mousseline sauce is hollandaise to which 50 g (2 oz.) heavy cream have been added. 

Double boiler preparation

1. in the upper part of a double boiler place the butter, egg yolks, vinegar and a pinch of salt; 
2. place over boiling water and stir constantly until thickened without letting the eggs curdle;
3. finish with the lemon juice and a little pepper;
4. if the sauce gets too thick, thin with a little hot water; 
5. for mousseline sauce, blend in the cream. 

1. melt the butter in the microwave;
2. combine all the ingredients in the blender; process for a few seconds to form a sauce;
3. with the blender running, add about 50 g (2 oz.) cream. If the sauce is too thick, add a little more cream.