How to Make Five Minute Microwave Coffee Cake in a Coffee Cup
For the crumb topping, put in a separate ramekin or bowl, 1 tablespoon butter, 2 tablespoons flour, 1 tablespoon of brown sugar, and 1 teaspoon of cinnamon, and smoosh it together with your fingers.
Add the crumb topping mixture to the cup that your coffee cake batter is in. Then microwave it. Start with 1 minute, then do 10 seconds more until it's done to your liking. For my microwave, a minute 20 seconds does it perfectly. It will be REALLY HOT when you take it out of the microwave. Let it cool down for a few seconds, then dig in.
Source: Prudent Baby