Tear the brioche bread into 1 to 1 ½ inch pieces and add to a large mixing bowl.
Add the eggs, milk and vanilla to a medium mixing bowl and whisk to combine.
Pour the egg mixture over the bread and gently stir with a wooden spoon to combine, making sure all the bread is covered. Refrigerate for at least 1 hour, or up to overnight.
Preheat the oven to 375 degrees.
Using a medium casserole dish, roughly 8”x10”, spray with cooking spray or grease with butter to prevent sticking. Spread half of the bread mixture into the pan.
Top with the peanut butter, one tablespoon at a time, evenly spaced on top of the bread.
Top with the quartered marshmallows, again evenly spread.
Add the rest of the bread mixture to the top of the casserole to cover the peanut butter and marshmallows.
Bake at 375 degrees for 35 to 40 minutes. A knife inserted into the casserole should come out clean.
Dust with powdered sugar and serve with syrup if desired.
Sourdough bread can be substituted for brioche, although brioche is ideal. The longer you soak, the better, with a minimum of 1 hour.Crunchy peanut butter can also be substituted for creamy peanut butter.If you are unable to find Stuffed Puffs marshmallows, which have a bit of chocolate in the middle, use regular marshmallows and a few chocolate chips. Stuffed Puffs also makes Chocolate marshmallows with chocolate inside and they are also very good in this casserole.Variations: chocolate marshmallows, pastel marshmallows for Easter, adding chopped peanuts, mini chocolate chips or chopped pecans.