Choose a 12- to 16-ounce microwave-safe mug with enough room for the bread and custard to expand slightly during cooking. Add the melted butter and swirl it around the bottom and lower sides.

Add the egg, milk, maple syrup, vanilla, cinnamon, and salt to the mug. Beat with a fork until the egg is completely blended and no streaks of egg white remain.

Add the cubed bread and gently press it into the custard with the fork. Turn the cubes so they are coated on all sides.

Let the mixture stand for 2 minutes. The bread should look moistened throughout but should still hold its shape. If using very dry bread, gently press the cubes down once more during the rest.

Microwave on high for 60 seconds: Check the center, then continue cooking in 15- to 20-second intervals until the egg mixture is set and the center is hot. Depending on microwave strength and mug shape, the total cooking time will usually be about 1 1/2 to 2 minutes.

The finished French toast should look puffed and set around the edges, with a soft but cooked center. If liquid egg remains in the center, microwave for another 15 seconds and check again. Avoid overcooking, which can make the bread rubbery.

Let the mug stand for 1 minute before serving; the mug and contents will be hot. Top with optional maple syrup, powdered sugar, or fresh berries.