Browned Butter Vanilla Sauce: In a small saucepan, melt the butter until it turns brown. To that, add brown sugar and milk. Cook over medium heat, stirring occasionally. In 5-8 minutes the mixture with thicken and comes to a full boil. Stir in vanilla carefully and let it cool.
Butter your mug of choice and place the bread cubes in it. In a small bowl, whisk together egg, milk, brown sugar, vanilla, and cinnamon. Pour it slowly over the cubed bread in the mug. Sprinkle with a pinch of nutmeg. Microwave on high for 1-2 minutes and check. You will want a toothpick to come out clean.
Pour the vanilla sauce on top and enjoy!