Render the bacon in a saucepan over high heat, stirring occasionally, for about 2 minutes. Add the shallots, garlic, and Armagnac and cook for 1 minute.
Stir in the glaze, sugar, salt, pepper, and pistachios and bring to a boil. Reduce the heat and simmer for 20 minutes. Whisk in the butter and remove from the heat. Serve immediately or prepare without the butter and store, refrigerated, in an airtight container for up to 24 hours. Reheat in a small saucepan over low heat. When the sauce is at a simmer, whisk in the butter, and remove from the heat.
Yield: 1 cup