Prepare the flavor base used in the classic method (that is, sauté the mushrooms, add the shallots, deglaze with Cognac and white wine, and reduce by half). Dissolve 3 tablespoons (45 milliliters) glace de viande in the flavor base. Thin the mixture to a light syrup by adding from ½ to ¾ cup (125 to 185 milliliters) concentrated (about six times) brown chicken stock. The base should have approximately the same consistency as the finished sauce. If you add too much stock and the base seems too thin, reduce it for a minute or two to bring it to the right consistency. It is preferable that the base be slightly thick rather than too thin; the sauce can be easily thinned at the end but cannot be reduced after the butter has been added.
Once the mixture has the right consistency, whisk in 1½ ounces (45 grams) cold butter and add 1 tablespoon (15 milliliters) chopped chervil and 2 teaspoons (10 milliliters) chopped tarragon. Bring the sauce to a simmer for about 20 seconds to give it sheen. Season with salt and pepper and correct the consistency by adding a small amount of chicken stock if necessary.