In a microwave-safe mug, add the gluten-free flour, coconut sugar, cocoa powder and a pinch of sea salt; stir to combine and break up any lumps.
Add the avocado oil (or melted coconut oil), water, ground cinnamon and vanilla extract; stir until the batter is smooth and even.
Stir in the chocolate chips or sprinkle them on top.
Microwave on high for 40–50 seconds, until the center is just set but still moist; avoid overcooking.
Let the brownie cool for 1 minute, then enjoy directly from the mug.