Mix Dry Ingredients: In a small bowl, whisk together the self-rising flour, cocoa powder, instant espresso powder, sugar, and cinnamon until well combined.

Add Wet Ingredients: Crack the egg into the bowl, then add the milk, vanilla extract, and vegetable oil. Stir until the batter is smooth and no lumps remain.

Prepare the Mug: Lightly coat the inside of a microwave-safe mug with cooking spray to prevent sticking.

Transfer and Cook: Pour the batter into the prepared mug, ensuring it’s no more than halfway full to allow room for rising. Microwave on high for 1 minute and 30 seconds. Check for doneness—the top should be firm but springy when lightly touched. If needed, microwave in 10-second intervals until done.

Garnish and Serve: Let the cake cool for 1-2 minutes, then dust with powdered sugar and enjoy warm.