The Dark 'N' Stormy is a rare bird indeed. It's a copyrighted cocktail. So if you want to make it, and you want to call it a Dark 'N' Stormy, you have to make it with Gosling's dark rum.
But here, we start (as with everything fun) from scratch. Let's make fresh ginger beer!
I used the recipe at Jeffrey Morgenthaler. Seemed like a good start:
2 oz ginger juice (juiced from 1 large ginger root)
4 oz fresh lemon juice, finely strained
6 oz simple syrup
20 oz warm water (cold if using the soda siphon)1⁄16 tsp Red Star champagne yeast
You'll need to sterilize your bottles. I picked up a chemical bottle sterilizant (some sort of peroxide, I think) at the brewing store, same place I picked up the packet of champagne yeast. Follow the directions to clean your bottles. I used the nice capped bottle shown above, because it'll handle the pressure of carbonation without exploding all over your home.
Funny aside: 10 years ago, I briefly got into homebrewing, and made some mead. Mead is fermented honey, and as such, it ferments rather slowly. Well I bottled 5 gallons of mead that I thought was done, and loaded up the wine bottles in my basement. Over the next few weeks, I guess there was a wee bit more fermentation going on in those bottles... One day, my roommate came home and slammed the door, just enough excitement to decarbonate a bottle. It blew the cork out, and sprayed carbonated mead all around the room. Well he rushed over and grabbed that bottle, knocking several more bottles... Pretty soon, there was a chain reaction of wine bottles blowing corks and spraying carbonated mead all around. When I arrived home, half my mead was gone, and he was not amused.
But that is not this story...
Okay, chop up a decent sized piece of ginger root.
And run that root through a juicer or food processor. Squeeze the ground ginger through a fine sieve to recover ginger juice.
It's an opaque yellow colour. Yum! Mix all the ingredients together, and pour into a pressurizable bottle. Wrong kind of bottle, and you'll be blowing carbonated ginger beer all over your home.
Let sit for 48 hours at room temperature, then put it in the fridge. You really don't want to go longer than that, or you'll explode the bottle.
Now comes the relaxing part. Mix 1 1⁄2 oz Gosling's dark rum with ginger beer to taste, over ice.