1tbspvanilla extract, the seeds from one whole vanilla bean, or 1½ tsp vanilla bean paste
½cup (112 g)powdered sugar
¼cup (39 g)cornstarch
Combine the dusting powder in a small bowl. Prepare a 7 by 11-inch or 9 by 13-inch pan by greasing it with butter and sprinkling it all over (including the sides) with the dusting powder and set aside.
Pour ½ cup (120 ml) cold water into bowl of stand mixer. Sprinkle gelatin over water and let set while you make the sugar syrup.
Pour the rest of the water (½ cup or 120 ml) into the bottom of a small saucepan. Add the corn syrup followed by the granulated sugar in the middle, trying not to get the sugar on the sides of the pan.
Cook over medium heat, without stirring, until the sugar dissolves and the mixtures begins to bubble. Clip on a candy thermometer and increase heat to high. Boil, without stirring, until the syrup reaches 240° F on the thermometer. Remove from the heat without agitating it too much.
With the mixer on low speed and fitted with the whisk attachment, carefully and slowly pour the hot sugar syrup into the dissolved gelatin, being sure to hit the lower side of the mixing bowl and not the whisk (or somewhere in between the two).
Whip until the mixture is very thick and the bottom of the bowl is only warm, about 10 minutes. Add the vanilla and mix thoroughly.
Pour the marshmallow mixture into the prepared pan and then sprinkle the top heavily with more dusting powder. Let set at least 8 hours or overnight.
The next day, tip out onto a silpat or large piece of parchment paper. It may require some coaxing to come out of the pan. Dust with more dusting powder and cut into long strips one way. Then turn and cut into 2-inch marshmallows. Place marshmallows in more dusting powder to coat and then shake in a sieve to remove excess powder. Keep in ziptop bag or a jar or container with a tight-fitting lid for up to 3 weeks.