Reduce 5 fluid ounces (150 milliliters) Mushroom Essence by half. Add 1¾ cups (450 milliliters) demi-glace to the reduction. Bring the mixture to a simmer and skim off any froth that rises to the surface. Strain through a fine chinois and whisk in 1 ounce (30 grams) butter. Finish with 2 ounces (60 grams) button mushrooms that have been simmered in a covered saucepan in ¼ cup (60 milliliters) water or light stock and drained. The cooking liquid from the mushrooms can be used to supply or augment the mushroom essence.