Prepare a 13x9 inch baking dish by coating with cooking spray or lightly greasing with butter.
Spread a rounded tablespoonful of peanut butter on 6 slices of bread. Spread 1 level tablespoonful of jam or jelly on the other 6 slices of bread. I leave the crust on! Place the two slices of bread together to make sandwiches.
With a serrated knife (steak knife will work), make one cut down the middle of each sandwich lengthwise. Then, make two crosswise cuts across the lengthwise cut. This will create 6 pieces.
Starting at the short end, arrange 6 pieces across the short end, with the cut side up. Repeat this procedure with the remaining pieces.
In a medium bowl, beat eggs with a fork or whisk. Gradually whisk in milk, then add sugar and salt. Drizzle this mixture slowly over the sandwich pieces until all pieces are coated. It’s ok if the mixture pools at the bottom. The bread will soak it up.
Place butter in a small microwave-safe bowl and cover with a paper towel. Microwave on HIGH for 25 seconds. Stir gently until the butter is completely melted. Using a pastry brush, brush or drizzle melted butter over the sandwich pieces.
Cover with aluminum foil and place in the fridge 8 hours or overnight.
Preheat oven to 325F.
Bake, covered with aluminum foil, for 30 minutes. Remove the aluminum foil and bake an additional 15 to 20 minutes or until the edges are slightly golden brown.
Sprinkle with powdered sugar or drizzle with maple syrup, if you like.