Stack 3 slices of bread together and trim off the crusts. Repeat with the remaining 3 slices.
Lay the bread slices on a chopping board or tray. Spread about 1 teaspoon of peanut butter onto one slice of bread.
Place another slice on top and spread 1 tablespoon of custard over it. Finish with the third slice of bread to create a layered sandwich. Repeat with the remaining bread slices to make a second sandwich.
In a shallow bowl, whisk together the eggs, milk, vanilla extract, and sugar.
Dip each sandwich into the egg mixture, coating all sides evenly.
Heat about 1 1/2 inches of neutral oil in a small deep pan over medium heat.
Carefully shallow fry the sandwiches for 2–3 minutes on each side until golden brown and crispy.
Remove and drain briefly on kitchen paper.
Serve warm drizzled with sweetened condensed milk and maple syrup or golden syrup. Top with a small slice of butter.