Toast the challah until it is dark brown, just shy of burnt. 

While the challah is toasting place one tablespoon of brown sugar, butter, and cinnamon in a small microwave safe bowl. 

Microwave on high for 30-60 seconds, or until the butter melts and the sugar starts to dissolve. Stir to combine. Spread the butter/sugar mixture over the toast in a thick even layer. Cut the toast into ¾ inch cubes.

Grease a 14-ounce, microwave safe, mug. 

Place the toast cubes in the mug. In another bowl combine the remaining tablespoon of brown sugar, milk, egg, vanilla, and salt, and whisk until well combined. Pour the egg mixture over the toast in the mug. Use a spoon to press the toast down to make sure all the cubes of toast get soaked with the egg custard. 

Microwave the mug for 1 ½ -2 minutes, depending on the strength of the microwave. The French toast is ready when a tester comes out without any liquid egg. Let cool for a couple of minutes and enjoy!