Homemade marshmallow is so very delicious, and so very short-lived. It just won’t hold when made well, without too much gelatin and no commercial stabilizers. So make it after dinner while everyone recharges for dessert, then eat it all up. Again, we’re turning to The Perfect Scoop for the basis of the perfect marshmallow sauce, one that uses hot syrup so that the egg white is “cooked” and safe for everyone to eat.
In a small bowl, sprinkle the gelatin powder over 1/2 cup cold water and set aside. In a small, heavy-duty saucepan fitted with a candy thermometer, mix the remaining 1/4 cup water with the sugar and corn syrup. Put the egg white and cream of tartar in the bowl of an electric stand mixer fitted with the whisk attachment.
Bring the sugar mixture to a boil, and when the syrup reaches 225 degrees F, begin beating the egg white at medium speed. When the syrup reaches 240 degrees F and the egg white is stiff (be careful not to over beat the egg white), remove the thermometer from the syrup and pour it into egg white in the running mixer in a slow, steady stream. Aim between the whisk and the side of the bowl.
While the marshmallow is whipping, scrape the softened gelatin into the warm saucepan (it will still have some residual sugar syrup in it; that’s fine). The heat of the pan will melt the gelatin off the heat. Pour the gelatin into the marshmallow while its whipping. Beat until the marshmallow is room temperature, then whip in the vanilla. Serve immediately.