A sugar syrup taken to the thread stage and beaten into dissolved gelatine until the mixture turns white and thick, then set in a tray dusted with powdered sugar and cut into squares. Two ingredients do the work: sugar and gelatine.
The thread stage is the whole recipe. Lift a little syrup on a spoon and let it fall: while it is still just sugar water it drops off, and at the right moment it stretches into a thin thread as it falls. That is the point to stop. Past it the syrup goes to caramel and the marshmallows set hard. Pour the syrup into the gelatine slowly, in a thin stream down the side of the bowl, with the mixer running. Tipping it all in at once cooks the gelatine where it lands and leaves lumps that never beat out. Then keep beating, longer than feels necessary: the mixture has to go from clear amber to opaque white and hold its shape. It also has to set properly before it is cut, and that takes hours rather than minutes, whatever the total time on the card says. The recipe does not give a number, so judge it by touch: when the surface no longer sticks to a clean finger, it is ready. Powdered sugar goes on the tray before the mixture and on the top after, and on the cut faces too, or the squares stick to each other in the box.