Freeze the bananas for 1 hour with the skin on, and slice as thinly as possible with an electric slicer. As they are sliced, place them on a nonstick rubber mat.
Preheat an oven to 75°C / 170°F.
Brush the bananas with simple syrup on both sides. Handle very carefully in order not to break the delicate slices. Once they have been brushed on both sides, remove the peel. The peel helps move the slices with little damage to the banana.
Place the nonstick rubber mat on a sheet pan.
Bake until the banana slices are firm, about 2 hours. Remove them from the rubber mat while they are hot and let them cool on a marble surface. If they cool on the rubber mat, it will be nearly impossible to remove them without breaking them.
Reserve in an airtight container at room temperature. If they are kept in a very dry environment, they can last for up to 1 week.