Split the vanilla beans down the middle lengthwise, leaving one end attached. You can scrape out the seeds and add them to the bottle if you like. Poke the beans split end down into the bottle.
Fill the bottle to the top with vodka. Wipe away any vanilla seeds from the top of the bottle, and put on the lid or stopper. Shake well.
Store in a cool dark place for at least 3 months (6 is better, if you can wait.) Shake the bottle as often as you remember to.
It's a good idea to label your bottle with the date you made it, so you know when to start checking it. If you're experimenting with different alcohols, different vanilla beans and different numbers of beans, then make a note of that too.
Once the extract is ready, I like to decant it into a smaller bottle to keep in the kitchen, and leave the bigger bottle, with the beans still inside to continue maturing in flavour. When putting the extract into a smaller bottle, you can strain out the seeds if you want to.
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Notes
This recipe makes 1 litre of vanilla extract. You can scale this up or down to whatever yield you'd like, just use 1 part rum to 3 parts vodka, and use at least 5 vanilla beans for every 250ml alcohol.If you want pure, unadulterated vanilla extract, just replace the rum with more vodka. Although in this case, I definitely suggest being an adulterer.You can experiment with flavours by replacing the rum with other liquors such as bourbon or even whiskey.