Two teaspoons of vanilla extract can be substituted for the vanilla bean; stir the extract into the cold custard in step 3. An instant-read thermometer is critical for the best results. Using a prechilled metal baking pan and working quickly in step 4 will help prevent melting and refreezing of the ice cream and will speed the hardening process. If using a canister-style ice-cream machine, be sure to freeze the empty canister at least 24 hours and preferably 48 hours before churning. For self-refrigerating ice-cream machines, prechill the canister by running the machine for 5 to 10 minutes before pouring in the custard. Cook time includes chill time.