In a microwave safe, 10-12 oz (or larger) mug, stir together the flour, sugar, cocoa powder, salt, and baking soda with a fork. Add in the water, oil, vanilla extract, and white vinegar, and stir together with the fork until smooth.
Microwave for about 1 minute, depending on your microwave, or until the top of the cake no longer looks wet. For me this takes 1 minute and 10 seconds, and I just give it a quick check at 1 minute to make sure. Top with a scoop of ice cream and enjoy!