Wow what a fascinating voyage of discovery. I have combined The Complete Confectioner recipe of 1864 for Syrup of Marshmallow with a recipe of my own for Italian meringue:
egg whites 5
sugar 280 g
marshmallow root 50 g
water 350 ml
Simmer marshmallow root in water for 10 minutes, blend softened root to a soup like consistency with a stick blender, simmer for 20 minutes. Add water if necessary to maintain a soup like consistency. Strain and add sugar to the brown / green liquid. Boil to the soft-ball stage (234F to 240F; 112C to 116C on a candy thermometer). Be careful in the last minutes that the froth does not rise over the pan top.
Whip egg whites to hard peaks. While continuing to whip add in the hot syrup. The mixture will expand and become light green brown. Scoop out mixture onto a dusted baking tray and dry in an oven.
What's amazing about the syrup is its stringiness and bounce. The old texts call it mucilaginous. It's quite possible to pull very fine strand 30 cm or more from the pan. The taste is also quite remarkable. I can confirm that vanilla is definitely not the original marshmallow taste. It's a complex taste with a nice lingering floral, nectar like finish. I detect notes of caramel, not fully cured vanilla bean and mushroom. Nothing quite like it. Fantastic.
I'll try and post some images of the finished marshmallows ifI have time and the kids don't polish them off too quickly.