Step 1 In a blender or food processor, puree the cherimoya until smooth and set aside.
Step 2 In a large saucepan, mix the sugar, egg yolks and milk together over medium-high heat, stirring constantly. Do not boil.
Step 3After about 30 minutes on the stove (the mixture will have thickened into a custard while simmering), remove the pan from the heat and pour the mixture through a cheesecloth into a bowl set into an ice bath. Stir as the mixture cools.
Step 4When the custard has cooled to 40 degrees, take it out of the ice bath and blend it in a blender with the cherimoya puree. You may do this in batches if your blender is small.
Step 5Freeze in an ice cream maker according to the manufacturer's instructions. Spread it evenly into a container with a lid, cover the surface of the ice cream with plastic wrap and then the lid and freeze it for at least 5 hours, or overnight. Makes 5 cups.