Peel and slice the bananas into 1-inch slices.
Spread the banana slices on a parchment-lined tray or inside a 1-gallon ziploc bag and freeze overnight or until solid.
Prepare the coffee syrup by mixing the instant coffee with the liquid sweetener. Let sit until the coffee is fully dissolved.
In a food processor or blender, blend the frozen banana slices and coffee syrup until smooth.
Serve immediately as soft serve, or pour into a glass or metal loaf pan, cover, and freeze for a firmer texture.