When I'm dead on my feet, my go to is Mr. black for the caffeine and Tequila for its staying awake powers. Add that to coffee and it's powerful enough to wake the dead. But it still needed a bit more, so some agave syrup to sweeten and stabilize the foam and the angostura bitters to play with the natural flavors in both the coffee and tequila. Since the angostura is shaken with everything, the flavor is woven into the foam to add to the experience.
Add all ingredients to a shaker (bitters included) with 2 to 3 cubes of ice
Shake vigorously. The idea is to froth up the coffee as much as possible.
Strain into a rocks glass with some ice.