Espresso powder (or crystals) is brewed espresso which has been dehydrated -- an "instant" version of espresso. In addition to using as a beverage, the powder is an excellent addition to any dessert that uses chocolate.
The espresso powder compliments and brings out the chocolate flavor. Our favorite pot de creme recipe uses instant espresso powder. It is also used in cakes, brownies, pies and other desserts.
A few years ago this was a very hard to find ingredient and we used to sell mountains of it for a few years but that has changed. Look for this ingredient at any well stocked grocery store or it is easily found online via KingArthurFlour.com on Amazon.com: Espresso Powder