Butter inside of a mug. Add 1 tablespoon butter, brown sugar, flour, cocoa, instant coffee, salt, baking soda, water, vanilla, and chips to the mug. Stir until completely combined.
Wipe inside rim of cup to remove excess batter. Microwave on High power until cake has risen and looks moist (not wet) on top, checking every 30 seconds, about 1 to 2 minutes. Avoid overcooking or it will be dry. Let rest to cool briefly before topping with ice cream and eating.