1/2 cup plus 1 tablespoon Armagnac
1 cup heavy cream
1 1/2 cups milk
6 large egg yolks
2/3 cup sugar
Have ready a large bowl of ice and cold water. In a 1 1/2-quart heavy saucepan simmer 1/2 cup Armagnac (being careful not to let ignite) until reduced by about half. Stir in cream and milk and bring mixture to a simmer.
In a bowl whisk together yolks and sugar. Add hot milk mixture to yolk mixture in a slow stream, whisking. Transfer mixture to cleaned saucepan and cook over moderate heat, stirring constantly with a wooden spoon, until slightly thickened and a thermometer registers 170°F (do not let boil). Pour custard through a fine sieve into a metal bowl set in ice water. Stir in remaining tablespoon Armagnac and cool custard completely.
Freeze custard in an ice-cream maker. Transfer ice cream to an airtight container and put in freezer to harden. Armagnac ice cream may be made 1 week ahead.