Homemade marshmallows:

You will need:
5 to 10 g agar agar powder
100 g cold water
250 g sugar
50 g icing sugar
50 g cornstarch
1 tablespoon honey
3 egg whites
1 pinch of salt

The original recipe mention to dissolve 5 g agar in 100 of cold water. I would double the agar agar because my marshmallows were a little bit too soft. Beat the egg whites until stiff with a pinch of salt. During this time (or in sequence if you don’t have a robot) make a syrup with about sugar, honey and water in which you previously dissolved the agar agar and boil for 7 minutes.

Pour syrup in thin stream over the egg whites while continuing to beat. Beat the whites and syrup together until cool (it may be long without a robot). Meanwhile, paper a rectangular mold with parchment paper. When cool, spread the white syrup in the pan, trying to smooth the surface and let stand overnight.

Next day, unmold and cut into small cubes as you ride one to one in a mixture of cornstarch and icing sugar. Eat alone or in a cafe mocha, hot chocolate or milk shake.

Recette originale.
Original recipe (in french).