First, it is important to point out that per the King Arthur Baking blog, espresso powder, in the minuscule amount that you will be using, does not add a coffee or mocha flavor to your baked goods, but rather adds intensity to your chocolate. That said, if you do not have espresso powder hiding in the dark corners of your pantry, do not try and use ground coffee. As America’s Test Kitchen notes, substituting ground coffee for the espresso powder will not only kill your brownie buzz, it will upset the delicate anticipation of the first chocolatey bite.
Why doesn’t ground coffee work? Because regular old coffee grounds will overwhelm the chocolate and leave you with brownies that not only taste like coffee, but make you feel like you have a mouth full of coffee dirt rather than lush, chewy chocolatey brownies. Instead, skip it and heed the advice of America’s Test Kitchen, which suggests using a little instant coffee or even ground espresso beans if espresso powder is not readily available.
But how much should you add? America’s Test Kitchen says you can use the same amount as you would if you had espresso powder. It recommends just 1½ teaspoons per 9×13-inch pan batch. However, King Arthur Baking’s blog says that if you use instant coffee in place of the espresso powder, you will want to use about 50 percent more, and there’s a possibility that it could add a bitter taste to your brownies.