Lightly grease an oven safe 9"×13" baking dish (22 cm x 33 cm).
Melt butter in a small pot over low heat. (Alternatively, melt in a microwave safe bowl in the microwave in 15 second increments, stirring in between, until melted.)
Add 1 cup brown sugar, mixing well to combine. (If on the stove, let simmer for 1 minute to slightly dissolve the sugar.)Pour mixture into prepared dish and spread out to evenly coat.
Beat together eggs, milk and vanilla. Set aside.
Combine remaining brown sugar, cinnamon and nutmeg in a separate bowl. Set aside.
Arrange bread in a single layer over the butter/sugar mixture in the dish.
Pour half of the egg wash over bread. Top with half of the brown sugar/cinnamon.
Arrange remaining bread slices over the top. Pour over remaining egg wash and top with remaining brown sugar/cinnamon. Lightly press bread into the egg wash to ensure slices are evenly soaked.
Cover tightly with foil and refrigerate for at least 30 minutes to soak up the custard mixture, or overnight for the best results
When ready to bake, take the dish out of the refrigerator 20 minutes before baking (if it has been in overnight) to get the chill out. Preheat oven to 350°F (175°C).
Bake, covered, for 30 minutes. Uncover and continue baking for a further 15 minutes.
Top with icing/powdered sugar. Serve with the syrup from the dish.