Place flour, sugar, cocoa, salt, and cinnamon in a microwave safe ceramic mug. Stir with a fork or spoon to mix well and break up any clumps

Add the oil, water, and vanilla to the cup and stir until the mixture is smooth and there are no lumps

Place in microwave and heat on high until the mixture is cooked through, about a 1 minute and 40 seconds for a 1000 watt microwave. You may have to experiment and adjust the time for less or more powerful microwaves. If you don't know the power level on your microwave, start with 60 seconds and increase until the brownie is done. It should still be moist when cooked through, not dry.

Let cool for a minute and serve with a scoop of vanilla ice cream or a teaspoon or two of whipping cream poured over.