Step 1Place the quartered prunes in a small saucepan with the Armagnac and 1 tablespoon of sugar. Heat over medium-low heat just until the Armagnac starts to bubble. Remove from the heat, cover and let stand at least 2 hours. (The prunes can be macerated a few days in advance -- refrigerate the prunes and Armagnac in a covered container or sealable plastic bag with the air removed.)
Step 2To make the ice cream, puree the prunes and their liquid in a food processor along with the sour cream, milk, the remaining 6 tablespoons sugar, lemon juice, vanilla and salt. Pulse the mixture until it's almost smooth, but leave a few little bits of prunes remaining.
Step 3Chill the mixture in the refrigerator for about an hour, then freeze in your ice cream maker according to the manufacturer's instructions. Makes about 1 quart.