Melt 2 tablespoons of the butter in a saucepan and sauté the shallot for approximately 2 minutes (or until transparent).
Deglaze the pan with the wine and reduce for 2-3 minutes.
Add demi glace, stir and simmer until the sauce is thick enough to coat a spoon.
Add the tarragon and finish with the remaining tablespoon of butter.
Season to taste with salt pepper and serve.