1) Cut butter up into pieces (around 8 to 10 pieces is good). Put the butter into the freezer.
2) Blend the rest of the ingredients together, with either a stick blender or standing blender.
3) Heat slowly in small saucepan or double boiler. Whisk or stir frequently. When curd begins to thicken to the point where it coats the spoon, or leaves a trail when the bottom of the pan is scraped with a spatula, or is 170 F, remove from heat (don't let it get to boiling).
4) Immediately whisk or stir in butter. Keep stirring until butter is melted and incorporated.
5) Taste. If you think it's too "sharp," try adding another Tablespoon or two of butter.
6) Put in airtight container. A piece of plastic wrap on the surface will keep a "skin" from forming. Cool completely before using (it will thicken a bit more).