Melt butter, brown sugar, and corn syrup in a small saucepan. Once melted, remove from heat and whisk to combine the mixture until smooth and fully incorporated. Pour into a 13 x 9 inch baking dish.
Add the cubed French bread on top of the sugar mixture in the baking dish.
In a bowl, whisk together eggs, eggnog, vanilla, and salt. Pour over the bread. Gently push bread down to make sure the custard covers all pieces. Cover the bread mixture and refrigerate for at least 4 hours. (Can make up to 24 hours in advance).
When ready to eat, preheat oven to 350 degrees. Bake uncovered until the bread is puffed and the edges are pale and golden, approximately 40 minutes.
Allow bread to sit for 5 minutes before serving. Sprinkle with powdered sugar and serve with warm maple syrup.
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Notes
You prep this dish up to a day in advance. Just cover and refrigerate until ready to bake. Day or two old bread works best here. Try to pick a hearty loaf from the bakery like an Italian, French, or sourdough loaf. Challah bread would also be a great choice. Leftovers can be frozen for up to a month or refrigerated for up to a week.