To heat-treat the flour: Microwave on a medium heat for 1 minute 30 seconds stirring frequently, then let it cool fully before using.
Mix all cake ingredients together in a mug.
In a separate bowl, mix the melted butter, light brown sugar, and cinnamon for the cinnamon swirl.
Layer the cinnamon swirl mixture into the cake batter.
Microwave the mug cake, watching it closely to prevent overflow.
