The only purpose of the espresso powder is to enhance the taste of the chocolate. You will not be able to taste the coffee, and it is too little to keep you up like what cup of coffee does. If you don't have it or is uncomfortable adding, then please omit.
This recipe yields 12-18 muffins. Fill 3/4 full for 18 regular muffins or full liner for large muffins. I mad large muffins here. Adjust baking time to suit.