Step 1

Put the butter in a mug 10-12 ounce microwave safe mug. Microwave for 20-30 seconds until melted.

Step 2

Add the flour, cocoa, sugar, and vanilla and whisk.

Step 3

Add the club soda and whisk until smooth.

Step 4

Microwave on medium high heat for about 2 minutes. Check to see if the cake is done. The top will be puffy and slightly firm. If not, continue microwaving in 15 second intervals until done.

Step 5

Let cool for a few minutes, top with whipped cream, ice cream, or powdered sugar.