In a cocktail shaker with ice, add 4 ounces kombucha, 1 ½ ounces rum, ½ ounce orange juice, and 1 ounce Cointreau. Gently stir the contents together (don't shake it) for 15 seconds to combine.
Take an orange slice and run it around the lip of the glass to give it an orange scent. Put a strainer on top of the shaker and slowly pour the contents into a martini glass.
Rim the glass with an orange slice and then hang it on the side or let it float on top.
There might be sediment at the bottom of the glass bottle, but don't shake the bottle of kombucha to try to mix it. It is carbonated, so if you shake it, it will spray everywhere when you remove the cap.
If you want to chill the martini glass, fill it to the top with ice and soda water and let it sit for a few minutes while you're making the drink.
Put the ice in the shaker first so it chills the rest of the drink ingredients while you're making it.
Since kombucha is carbonated, don't shake the drink. It's best to stir it to combine. If you do shake it, it will get very fizzy and then you'll lose the bubbles.
Finish the drink by rimming it and garnishing it with a slice of citrus. You can hang it on the side or let it float on top.