Update: I tried this with Gosling's Black Seal rum and it is 1000% amazing. This might honestly be my favorite cocktail in the world. The coffee and the dark rum marry to an intense caramel flavor. You can put salt directly in the cocktail shaker -- no need to fuss with salt solution if you don't want to.
It's flattering to read this. Haven't searched for this cocktail on the web for a year and I should've done that earlier. Thanks for testing with other Rums, Gosling's must have worked beautifully. Cheers!