Grease the inside of an 9x13” baking dish with butter.
Build 8 peanut butter and jelly sandwiches, using about 1/4 cup of peanut butter and 1/4 cup of jelly per sandwich. Cut each sandwich in half on a diagonal.
In a large mixing bowl, whisk together eggs, milk, cinnamon, vanilla and salt until well-combined.
Arrange the peanut butter and jelly triangles cut-side down in the baking dish, staggering the crusts. Pour the egg mixture over the top of the sandwiches, cover with plastic wrap, and refrigerate overnight to allow the sandwiches to soak up the custard.
The next day, preheat oven to 350°F. Remove plastic wrap from the baking dish and cover with foil. Bake covered for 45-50 minutes or until almost cooked through. Remove foil and bake for 10-15 more minutes or until the top is browned.
To serve, spoon out individual triangles, about 2 per person, and garnish with powdered sugar, maple syrup and chopped peanuts.