Put the apples, butter, and water in a heavy saucepan. Add the vanilla bean, cover the pan, set it over medium heat, and simmer until the apples are completely tender and have cooked to a rough puree, stirring them gently from time to time, 20 to 25 minutes. Add the sugar (see notes), stir, and continue cooking uncovered for a few minutes to let the sugar dissolve. Taste the sauce for sweetness; it should be tart-sweet, but if you want it sweeter, add more sugar. Stir in the brandy and simmer a few minutes more to cook off a little of the alcohol. Remove from the heat and set aside to cool. Leave the vanilla bean in the applesauce until ready to serve; remove it before serving. If not serving the same day, refrigerate for up to a week.