Cut the bread into cubes. Add 1 tablespoon of butter to the bottom of a mug and microwave it for 20 seconds. Spread the butter around the bottom and up the sides, then add the bread chunks to the mug. The bread will cook down when you microwave it, so you want the mug to be slightly overflowing with bread cubes.
Add ¼ cup of milk, the vanilla, cinnamon, nutmeg, coconut sugar, chia seeds, and a pinch of salt to a small bowl and whisk to combine. Let the mixture sit for 5-10 minutes, stirring frequently. This mixture will replace an egg, and the chia seeds will slightly gel up. Pour the mixture over top of the bread cubes in the mug.
Add the additional tablespoon of milk over top and microwave for 1-2 minutes. You want to make sure the bread is soft but not overcooked, so start with 1 minute 30 seconds and then another 30 seconds after that, if needed. Remove the mug from the microwave and let cool slightly.
Meanwhile, to make the topping, melt a tablespoon of butter, ¼ teaspoon of cinnamon, and a pinch of nutmeg in a small bowl. Pour that over top of the cooked bread cubes and let sit for a couple minutes, then pour maple syrup over top and serve.