Put the rhubarb in a 2-litre sterilised preserving jar or bottle, then pour in the vodka and sugar. Add the vanilla pod, if using.

Mix until the sugar has dissolved, then seal the jar or bottle and leave to infuse in a cool, dark place for two months. Strain through a muslin cloth or fine sieve and decant into a bottle before using. Will keep in a cool, dark place for up to six months.