The Modernist Cuisine Baking Steel, placed atop a 10-pound block of dry ice, can cool down from room temperature to 15 degrees in 10 minutes. Thus, you can quickly make ice cream with personalized mix-ins without setting up an ice-cream maker or procuring liquid nitrogen.
Place the baking steel in the freezer until it reaches 15 degrees. Alternatively, place the steel on a block of dry ice.
Mash the banana into a paste on the steel. Scrape and flip the mash frequently to prevent it from sticking to the steel surface.
Mash the kiwi into the banana paste, and cut, press, and flip the mash until mixed well. Push the mixture to the far edge of the plate. Place the spatula, inverted, on top of the mixture, and then drag it downward to spread the mash into a thin layer.
Run the spatula left to right over the street until the layer of mash becomes thin and uniform. Wait 20 seconds for the layer to freeze, and then use the spatula in its normal orientation to scrape it off the steel in one smooth, swift motion from front to back. The frozen fruit will naturally curl into a roll.