Prepare the mug by spraying the inside with non-stick cooking spray or wiping with a little butter to prevent sticking.
Mix the dry cake ingredients directly in your mug—flour, sugar, baking powder, and salt—until well combined with no lumps.
Add wet ingredients to the mug (milk, oil, and vanilla) and stir thoroughly with a fork until the batter is smooth and no dry spots remain.
Create the cinnamon swirl mixture in a separate small bowl by combining melted butter, brown sugar, and cinnamon until it forms a paste-like consistency.
Drop spoonfuls of the cinnamon mixture onto the cake batter and use a knife or toothpick to swirl it throughout the batter, creating that classic cinnamon roll pattern.
Microwave on high for 60-90 seconds, watching carefully—cooking times vary by microwave wattage. The cake is done when it rises and looks set but still appears slightly moist on top.
Let the cake cool for 1 minute while you mix the glaze ingredients together until smooth.
Drizzle the glaze over the warm cake and enjoy immediately while it’s still warm!