Mix all ingredients together, except butter, in a double boiler nor heavy saucepan.
Cook until thickened, stirring constantly to ensure it does not scorch.
Remove from heat, add butter.
If lumpy, smooth in a blender or with a hand mixer.
Notes
Makes a 750 ml jar. The recipe can be altered easily to make a half batch or double batch.
Use this mayonnaise to compliment your salads, etc. by mixing this mayonnaise with regular store-bought mayonnaise or salad dressing: I use 1 part of this mayo to 4 parts commercial mayonnaise, or to taste.