Tear the bread slice into small pieces and place them in a microwave-safe mug. Use a fork to lightly mash some pieces, creating an even base for mixing.
In a small bowl, whisk together the egg, milk, cinnamon, and a pinch of salt until the mixture is uniform and slightly frothy. Pour this mixture over the bread in the mug.
Use a spoon or spatula to gently push the bread down, ensuring all pieces are immersed in the liquid. Let sit for 30 seconds to allow the bread to soak up the mixture.
Place the mug in the microwave. Microwave on high for 1 to 1.5 minutes, or until the surface is set and the edges slightly pull away from the mug. You should hear a gentle bubbling sound toward the end.
Remove the mug carefully from the microwave. Check doneness by inserting a toothpick or knife; it should come out clean. Let rest for a few seconds before serving.
Optional: drizzle with honey or sprinkle with extra cinnamon. Serve warm with a spoon, revealing a fluffy, golden interior with a lightly crispy top.
