Candied Chestnut and Vanilla Ice Cream

1 qt vanilla ice cream softened
1 cup finely chopped marrons glaces chestnuts (candied available at specialty stores)
1 1/3 cups heavy cream
1/3 cup firmly packed dark brown sugar
6 oz fine-quality bittersweet chocolate chopped fine
1 oz unsweetened chocolate chopped fine
2 tbl unsalted butter
2 tbl Cognac or to taste

In a bowl stir together well the ice cream and the marrons glaces and freeze the mixture, its surface covered with plastic wrap, for 2 to 3 hours, or until it is firm. In a heavy saucepan boil the cream, stirring occasionally, until it is reduced to about 1 cup, whisk in the brown sugar, and simmer the mixture, whisking, until the sugar is dissolved.

Remove the pan from the heat, whisk in the chocolates, whisking the mixture until it is smooth, and whisk in the butter and the Cognac. Serve the sauce warm over the ice cream.