Whiskey is a two-edged sword in cooking. On one hand, it has thousands of flavonoids and esters that come from the oak barrel, fermentation grains and aging, and thus has a combination of woodsy, spicy, fruity and floral aromas and flavors to work with. On the other hand, it has ethanol, which doesn't work well in cooking, especially in large doses. To make a savory whiskey sauce, you must extract the most flavor and also burn off the ethanol. In sweet whiskey sauce, you have to build a creme anglaise base and add a minute amount of whiskey to taste.
Pour the milk in a heavy-bottomed saucepan and heat it on the stove over low heat; you need about 1 cup of milk for each batch of sauce. Add aromatics and flavoring, such as allspice berries or vanilla beans, to the milk, if desired.
Whisk 2 parts pasteurized egg yolks and 1 part sugar until it turns a pale yellow color; you need 4 yolks and 2 tablespoons of sugar for 1 batch of sauce. Drizzle in the hot milk while whisking vigorously.
Transfer the egg yolks and milk to the heavy-bottomed saucepan and set the heat to medium-low. Cook the creme anglaise until it coats the back of a spoon, about 5 to 6 minutes.
Fill a mixing bowl with equal parts ice and water. Set a fine-mesh strainer over a mixing bowl and strain the creme anglaise through it.
Add a scant pinch of salt and whiskey to taste. Start with about 1 teaspoon of whiskey per cup of sauce and add more as needed.
Saute diced aromatics in a few tablespoons of butter in a saute pan over medium heat until caramelized. Onions, garlic, mushrooms, scallions, shallots, peppers -- anything pungent -- works in a whiskey sauce. If you're cooking a pan-roasted steak, take it out of the pan when it finishes and add the butter and aromatics to the drippings.
Take the pan off the stove and pour in the whiskey; you need about 1/4 cup of whiskey for each batch of sauce. Ignite the whiskey with a match.
Return the pan to the heat when the flame subsides. Scrape the bottom with a wood spoon or scraper and simmer the whiskey until reduced and syrupy.
Spoon 1 tablespoon or so of Dijon or whole-grain mustard in the sauce along with twice as much heavy cream. Stir the sauce and cook until it comes to a simmer.
Add beef stock to the sauce to taste and bring it to a simmer. Taste the sauce and adjust the seasoning with kosher salt, freshly ground black pepper and Worcestershire sauce. Stir in freshly chopped herbs and serve.