Espresso powder is very intensely dark and concentrated instant coffee. It's not just coffee beans ground fine. It's actually coffee crystals that dissolve quickly in liquid.
Espresso powder is really not for drinking - bakers are the primary people who use it. It's also different from plain old instant coffee in that it's much more concentrated. Espresso is, after all, a form of coffee - not a different kind. Espresso grounds are darkly roasted coffee, ground very fine for the espresso extraction.
Instant espresso has a better, darker flavor than your average storebought instant coffee. Just a teaspoon will give a darker, richer flavor to your chocolate recipes; it won't make them taste much like coffee but it will enhance their flavor. Using more than a teaspoon starts to bring out coffee flavors.
It's not usually available in the grocery store; you need to look for it in specialty stores or online. Sur La Table usually carries it.
But what if you don't have it and you don't have time to hunt it down? It's perfectly acceptable to use instant coffee instead. Use about 50% more than the amount of espresso powder called for in the recipe, but taste as you. Instant coffee usually has a harsher taste than instant espresso; it can taste tinny or sour if too much is used.