In a mixing bowl whisk the eggs and egg yolk until combined. Add the brown sugar, vanilla, espresso powder, salt, coffee, and whiskey. Then whisk in the milk until all is blended.
4 eggs, 1¼ cups whole milk, 1 cup brown sugar, ½ cup coffee, ¼ cup Irish whiskey, 1 egg yolk, 1 tablespoon espresso powder, ½ tablespoon vanilla, ½ teaspoon kosher salt