Preheat your steam oven (or a sous vide circulator in water bath) to 135°F/57°C. If you're using a steam oven, use the sous vide setting if your oven has one, or the steam setting if not.
Put the vanilla beans and vodka into a wide mouth or mason jar and close the lid to fingertip tightness (screw the lid on to the point where you've tightened it with your fingers, but don't twist it on as hard as possible, or you run the risk of the glass cracking during the heating and cooling process).
2 oz vanilla beans, 2 cups vodka
Put the jar into your steam oven or water bath and set the timer for 96 hours/4 days. Yes, that's a long time! If you're using an unplumbed (water tank) steam oven, you will need to periodically fill up the water tank during this time, probably about twice a day.
At the end of the cooking time, cool the vanilla extract to room temperature and store in a dark place in the pantry.
The extract is ready to use when it's a dark, deep brown color and the alcohol smell has mellowed, anywhere from 1-3 months. If you want to do a taste test, put a couple of drops of extract into a tablespoon of milk. If it's ready to use you'll get a lovely deep vanilla taste without an alcohol 'burn' on your palate.