Combine the gelatin and 1/2 cup of cold water in the bowl of an electric mixer fitted with the whisk attachment. Allow to sit while you make the syrup.
Meanwhile, combine the sugar, corn syrup, salt, and 1/2 cup water in a small saucepan and cook over medium heat until the sugar dissolves. Raise the heat to high and cook until the syrup reaches 240 degrees F on a candy thermometer, then remove from heat.
Start with the mixer on low speed, and slowly pour the hot sugar syrup into the dissolved gelatin. Increase the mixer speed to high and whip until mixture is very thick and stiff peaks form, about 15 minutes. Mix in the vanilla and ground Earl Grey.
In a small bowl, combine the powdered sugar and cornstarch.
With a sieve, generously dust an 8- by 12-inch baking dish with the powdered sugar/cornstarch mixture. Pour the marshmallow mixture into the pan, smoothing the top with a greased rubber spatula. Dust the top with the remaining powdered sugar/cornstarch mixture. Allow to stand in the fridge uncovered overnight until the marshmallows are dry to the touch.
Turn the marshmallows onto a board and cut them into desired shapes. Dust again with more of the powdered sugar/cornstarch mixture. and enjoy!
Store in an airtight container and refrigerate for up to two weeks.