Build in glass, Collings or highball, filled with ice.
Pour in ginger beer and lime juice.
Pour in black rum on top, do not stir.
Serve while still cloudy-looking, garnish with lime wheel.
Notes
I scaled down the recipe for this as I used a smaller glass. I used 1 oz rum, 2 oz ginger beer, 0.25 lime juice. The ratio is the same, just the size is different.
It's called dark and stormy as a nod to the cloudy look it gets when you pour in the dark rum. Sometimes it doesn't mix as it sinks, and looks like billowing storm clouds.
Do stir right before drinking, the no-stir is there just for show.