Allow the bananas to sit out for a few minutes to thaw slightly. (You don’t want icy banana bricks in your processor.) Throw everything in a food processor and pulse until completely smooth. Freeze. ;)
I used unsweetened Torani coconut syrup which is made with sucralose, but there are also brands made with stevia, or you can use the Torani cane sugar version. This is the type of syrup you would use to sweeten coffees or frappucinnos.