Prepare the bread: Smooth a thin layer of mayonnaise on one side of each slice of bread. This will create a perfectly crispy golden exterior when grilled. On the opposite sides, spread the peanut butter evenly across all four slices. The peanut butter acts as the creamy base for your sandwich.
Add the banana: Place banana slices evenly over the peanut butter on two of the bread slices. Ensure the bananas are thinly sliced for even distribution and better melding of flavors during cooking.
Assemble the sandwiches: Take the two bread slices with just peanut butter and place them peanut butter-side down onto the banana-topped slices, creating two complete sandwiches. The mayonnaise-coated sides should be facing outward.
Grill the sandwiches: Heat a griddle or non-stick pan over medium-high heat. Place the assembled sandwiches on the hot surface with the mayonnaise side down. Cook for about 2 minutes, or until the bottom is golden brown.
Flip and finish cooking: Using a spatula, carefully flip the sandwiches and cook for an additional 2 minutes, or until both sides are golden brown and the filling is warm throughout. The peanut butter should be slightly melted and the bananas softened.