Hi Rachael...
There are a # of ways to make Beurre Blanc,some with cream some with out etc.Heres a basic way to make your sauce and should hold fine...Temputure and holding are keys to a great BB.lets say for four people,mince one shallot and put it in a sauce pot..to that add 2 sprigs of fresh thyme,a couple pepper corns and a bay leaf. Add 3 cups of dry white wine and one cup of white wine vinager (champange vinager is nice)start to reduce this slowly.In another high walled sauce pot add four cups heavy cream and start to reduce.have a half a pound of unsalted butter cubed and at room tempurture.When the cream starts to simmer add a pinch of saffron threads,kosher salt and white pepper,let simmer a couple more minutes and add a 1/2 cup pernod.Know Rachael,at this time you are looking for the wine/vinager reduction to be almost completly reduced..perhaps a table spoon or two of liquid on the bottom. Add the cream mixture to the wine reduction,reduce this down until it you can coat the back of a spoon,run your finger through the sauce and it holds...At this time lower your heat to low and add 1 cube of butter at a time and whisk with a ballon whip continuesly until all the butter is melted,strain and keep warm.Make sure it is not kept higher then 120 degrees when being held.Ontop of a gas stove works well...I hope this helps you.
cc