Espresso is not a type of coffee bean. Instead, it describes a method of making coffee that cannot be replicated with instant ground coffee. You can increase the ratio of instant coffee powder to water to concentrate the flavor for use in place of espresso in recipes or fancy coffee drinks, but technically it won't be espresso and won't taste the same.
Definition of Espresso
The concentrated shots of coffee known as espresso, which are served plain, with a dollop of cream or mixed into a latte or mocha, result from a specific process. Espresso comes from a machine that uses high pressure to force hot water through tightly packed, finely ground coffee in a short period of time, resulting in a dense, thick spot of joe with a layer of foam on top. Instant coffee requires you to only stir in hot water until the grounds dissolve. This process in no way replicates the art of espresso making.
What's Missing in Instant
Other methods of pressed coffee such as French press or moka pot may create a more robust cup of coffee than drip, but their strength doesn't make them espresso. You can achieve a strong cup of coffee by concentrating regular instant coffee, but it will lack the trademark foam, called crema, essential to a proper shot of espresso.
The first versions of espresso that appeared in the United States came from beans in Southern Italy, where primarily dark-roast beans were used, but espresso can be from any bean roasted from light to dark. A method of roasting that brings a lot of the beans' oil to the surface is a hallmark of espresso. The natural oil provides espresso with a distinctive mouth feel and body not present in instant coffee. Fresh beans stored in an airtight container away from light and heat should be used to create espresso. Coffee beans start to degrade 24 hours after grinding, so instant coffee cannot provide an equally deep aroma and fresh flavor of a cup of true espresso.
One product sometimes called for in recipes is instant espresso powder, which consists of intense, finely ground, quick-dissolving coffee crystals. It's more concentrated than regular instant coffee so it lends a richness to baked goods, especially those of the chocolate variety. The powder is not intended to be a substitute for drinking espresso, however.
Subbing Instant for Espresso
If you aren't so concerned with the look and authenticity of your espresso and just want the rich coffee flavor for your brownies or chocolate cake, instant coffee will do. If your recipe calls for espresso powder, substitute about 50 percent more instant coffee. Be wary, though -- adding too much instant coffee, which usually has a greater acidity, may result in a baked good with a slightly sour or metallic aftertaste.