Technique
Fill a highball with ice, combine ginger beer and lime, then pour the dark rum slowly over the back of a spoon if you want a defined float. For a fully integrated drink, add the rum first and give everything one gentle stir.
A Dark n Stormy is a highball cocktail made with dark rum and ginger beer, served on the rocks.

TonicDB editorial guide
A Dark and Stormy pairs spicy ginger beer with the molasses and barrel notes of dark rum. Building the rum near the top creates the storm-cloud look, while fresh lime prevents the sweet, intense flavors from becoming heavy.
Fill a highball with ice, combine ginger beer and lime, then pour the dark rum slowly over the back of a spoon if you want a defined float. For a fully integrated drink, add the rum first and give everything one gentle stir.
Serve in a clear tall glass so the layered version remains visible, with one lime wedge at the rim. Let the drinker stir before sipping if the rum was floated; the dramatic presentation is not meant to make the first sip straight rum.
Ginger ale lacks the spice and body expected here, and warm ginger beer loses carbonation quickly. An overly large rum float can also make the opening taste harsh even when the lower portion is balanced.
Use a particularly spicy ginger beer for a drier impression, or add a dash of aromatic bitters for extra spice. Keep the listed rum, lime, and mixer amounts steady before changing more than one of those elements.
These notes explain how to execute and adapt the recipe shown above. Ingredients and serving traditions vary, so taste and adjust thoughtfully.
Contributed by @alfg