Measure and cut the bread into small, bite-sized chunks and place them in a microwave-safe mug, spreading evenly.
In a small bowl or measuring cup, whisk together the egg, milk, cinnamon, vanilla extract, and a pinch of salt until well combined and slightly frothy.
Pour the egg mixture over the bread chunks in the mug, pressing down lightly to ensure the bread absorbs the custard. Let sit for about 30 seconds if needed to soak in.
Dot the top of the soaked bread with a small piece of butter or drizzle of melted butter. Microwave the mug on high for 2-3 minutes, checking for doneness by inserting a toothpick or knife into the center; it should come out clean or with moist crumbs.
Once cooked, carefully remove the mug from the microwave. Let it sit for 30 seconds, then top with syrup, berries, or powdered sugar if desired. Serve immediately with a spoon.
1 slice bread (white or sandwich bread)