In a large non-reactive saucepan, combine the milk and cream. Scrape the vanilla seeds into the pan, add the vanilla bean and bring to a gentle boil. In a small mixing bowl, whisk together the egg yolks, sugar, bourbon and Cognac. Discard the vanilla bean. Whisk 1 cup of the hot cream mixture into the egg mixture. In a slow, steady stream, gradually add the egg mixture to the hot cream mixture. Cook over moderately low heat, stirring occasionally, until the mixture thickens enough to coat the back of a spoon and reaches 170 degrees F on an instant-read thermometer, about 5 minutes. Remove from the heat and allow to cool.
Cover with wax paper or plastic wrap, pressing down against the surface to keep a skin from forming. Chill in the refrigerator for at least 2 hours.
Remove the custard from the refrigerator and pour into an ice cream machine. Churn and freeze according to manufacturer’s instructions. Transfer to an airtight container and spread, while still soft, onto the cake. (Alternately, freeze the ice cream until ready to use. Then allow to come to room temperature before using in the Buche de Noel recipe, so it can be spread easily on the cake.)
Yield: 4 cups