Split vanilla bean. In a small heavy saucepan bring milk and vanilla bean halves just to a boil and remove pan from heat. Scrape seeds from vanilla bean halves into milk and reserve pod for another use.

In a bowl whisk together yolks and sugar. Add milk in a stream, whisking, and transfer custard to pan. Cook custard over moderately low heat, stirring, until thickened slightly and a thermometer registers 170 degrees. Remove pan from heat and stir in Calvados. Pour custard through a fine sieve into a metal bowl set in a large bowl of ice and cold water and cool, stirring occasionally, until cold.

Sauce may be made 1 day ahead and chilled, its surface covered with plastic wrap.

Apple