Add heavy cream. Simmer for 2 or 3 minutes to reduce slightly.
Turn heat to low.
Add butter, a few pieces at a time, whisking constantly, to blend the butter into the reduction. The heat should be quite low as you work. Note: You will continuously have to monitor and adjust the heat as you add the butter. If you can see steam coming from the sauce the heat is good. Do not let sauce come to a boil. Continue adding butter until the full amount has been incorporated.
Taste. Add salt and pepper, to taste.
Set aside.
Can be done in advance. Store in a tightly sealed container, in the refrigerator, until ready to serve and any leftovers, up to a week. Reheat to warm before using.