Put the butter in a mug 10-12 ounce microwave safe mug. Microwave for 20-30 seconds until melted.
Add the flour, cocoa, sugar, and vanilla and whisk.
Add the club soda and whisk until smooth.
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.
Let cool for a few minutes, top with whipped cream, ice cream, or powdered sugar.
Notes
*The right size mug makes a difference. 10-12 ounces is perfect.
*I’ve made this recipe with store-bought self rising flour, as well as my own homemade version. The store-bought works infinitely better — don’t bother with a DIY version.
*If your club soda is at room temperature, the cake will rise much better.