Wash the rhubarb and discard the green leaves, as they are toxic. Chop the rhubarb and pack it into the jar. Pour the vodka over the rhubarb, all the way to the top. Screw on the lid, set in a cool, dark place (like your pantry), and let steep for 1 month.
To make the simple syrup, combine the water and sugar in a saucepan, bring to a boil to dissolve the sugar, and then turn off the heat. Strain the rhubarb-infused vodka, and discard the rhubarb. Pour the vodka and simple syrup back into the jar, and let rest for 1 month.
The last step is to filter the rhubarb cordial, to remove any impurities for a crystal clear liqueur. You can do this simply by setting a coffee filter over a container, pouring in the cordial, and letting it drip through.
After filtering the liqueur, pour it into pretty bottles. It's ready to enjoy!
The rhubarb cordial does not need to be refrigerated, as the high alcohol and sugar content will preserve the liquid, but you can refrigerate it if you want to drink it chilled.You can drink it straight, or add it to cocktails. It's delicious mixed with Sprite and crushed strawberries, or mixed with lemonade, champagne or white wine.