With a knife halve vanilla beans lengthwise. Scrape seeds into a large heavy saucepan and stir in pods, cream, milk, and sugar. Bring mixture just to a boil, stirring occasionally, and remove pan from heat. In a large bowl lightly beat eggs. Add hot cream mixture to eggs in a slow stream, whisking, and pour into pan. Cook custard over moderately low heat, stirring constantly, until the custard thickly coats the back of a spoon, or until a thermometer registers 170 degrees F. (Do not let boil.) Pour custard through a sieve into a clean bowl. Place the bowl in a large bowl of ice water and let the custard chill, stirring occasionally. Freeze custard in an ice-cream maker, in 2 batches if necessary. Transfer ice cream to an airtight container and put in freezer to harden. Ice cream may be made 1 week ahead.
Variation 1: To make chocolate ice cream, put 1/2 pound of finely chopped bittersweet chocolate in a large bowl, and pour the hot thickened custard over the top. Let sit for 1 minute, and stir until smooth. Chill and freeze as above. Variation 2: To make strawberry ice cream, hull and slice 1 pound (1 1/4 pints) fresh strawberries, puree in a food processor. Transfer the puree to a bowl, stir in 1/2 cup sugar and 1 tablespoon lemon juice, and chill. Add the puree to the chilled custard and freeze as above.