Reduce the chicken stock in a saucepan by one half.
In another pan sauté the shallots in 1 ounce of the butter. Return the remaining butter to the fridge until needed.
Add the vermouth and reduce to a syrupy consistency.
Add the cream and reduce by half.
Add the reduced chicken stock.
Stir in the tarragon and mustard.
Lower the heat and whisk in the remaining butter in increments until incorporated.
Season with salt and white pepper.