Add peanut butter to bread, and remove crusts: Spread 1 tablespoon peanut butter evenly over each of 4 bread slices. Stack 2 peanut butter–covered bread slices on top of each other and top with 1 remaining bread slice (without peanut butter); repeat with remaining bread. Using a sharp serrated knife, trim off outside crusts to create an even square shape.
Make egg mixture: In a pie plate or other shallow dish, whisk together milk; powdered peanut butter, if desired; and eggs. Dip each bread stack into egg mixture, turning to coat on all six sides.
Melt butter, and cook French toast: Melt butter in a medium skillet over medium-low heat; swirl pan to coat. Add bread stacks to pan; cook until browned on top and bottom, about 3 minutes on each side. Balance bread stacks to brown their edges, 1 to 2 minutes per edge.
Top with sweetened condensed milk, and serve: Arrange 1 French toast stack on each of 2 plates and top each with 1 tablespoon sweetened condensed milk; cut in half with a serrated knife.
