In a microwave-safe mug, place 1 tablespoon of butter. Heat in the microwave for 30 seconds until melted (not boiling).
Add the sugar and brown sugar to the mug and mix until well combined, about 1 minute. Mix in the egg yolk and vanilla just until combined, about 20 seconds. Add the flour, cocoa powder, espresso powder and salt, and mix just until combined. Stir in the chocolate chips, leaving a few extra to sprinkle on top.
Place the mug in the microwave and heat on full power for 40 to 60 seconds. Check at 40 seconds to see if the cookie is cooked in the center. Add more time if necessary (mine took 50 seconds). Serve warm.
Espresso powder substitute: If you don’t have a coffee bean grinder, substitute the espresso powder with ½ teaspoon to 1 teaspoon strongly brewed coffee. This works especially well at high-altitude where ingredients are dryer and baked goods need more moisture.Dry cookie in a mug: Try decreasing the time in the microwave to 40 seconds. Remember, the cookie will continue baking a little bit after it’s taken out of the microwave because the mug is still hot. If the cookie is still dry, consider adding 1 teaspoon of strongly brewed coffee in place of the espresso powder.Boyer’s Rocky Mountain Coffee is available in stores nationwide. Use their store locator or visit their website.