My thoughts and inspiration come from the wonderful juniper flavor in a gin and tonic cocktail. I believe the espresso should be the after taste, just like the juniper can be in a gin and tonic. Therefor I use just a splash of espresso.
1 oz Island Oasis Moscow Mule
4 oz tonic water
0.25 oz espresso
1 cup ice
Float espresso on top last, allowing guest to stir.
Pro Tip: Fresh espresso that is bright and not smoky is preferred. Best if using fresh espresso so it has crema and fizz, can pre-pull espresso and store in cool space
8 oz. beverage