Prepare the custard mixture by whisking together the beaten egg, milk, vanilla extract, cinnamon, sugar, and salt in a small bowl using a fork or whisk.
Add the bread pieces into the mug and pour the custard mixture over them, pressing down gently to ensure all bread absorbs the liquid. Let sit for 30 seconds for soaking.
Place the mug in the microwave and cook on high for 1 minute. Remove and check if the edges are golden and the custard is slightly set.
Stir gently to distribute the custard and ensure even cooking. Cook again on high for another 30 seconds to 1 minute, or until the top is golden brown and the custard is fully set. When done, insert a toothpick or skewer in the center; it should come out clean.
Remove the mug from the microwave carefully. Let sit for 30 seconds to cool slightly. Optionally, add toppings like fresh fruit or syrup before serving.