In a large saucepan over medium-high heat, combine 3/4 cup water, sugar, corn syrup, and salt.
Meanwhile, in the bowl of a stand mixer (or in a large mixing bowl), place 3/4 cup water and sprinkle gelatin on top; let plump.
Insert candy thermometer into sugar mixture, making sure not to touch it to the sides or bottom of the pan. Bring sugar mixture to a boil and let keep rolling boil until mixture passes 235 degrees F, about 3-5 minutes.
With mixer on low, carefully pour sugar mixture into mixer bowl and slowly raise beater to high speed. Be careful at this step because if you go up too high the marshmallow mixture will start spraying everywhere.
Beat sugar mixture on high for 5 minutes and then add vanilla. If you'd like to add other flavors here like peppermint, cocoa powder for chocolate, or color, add now.
Continue beating mixture an additional 5 minutes.
Meanwhile, oil a 13 x 9-inch pan for large marshmallows or two 10 x 10-inch pans for small marshmallows. Dust the pan with powdered sugar liberally.
When done beating, pour sugar mixture into pan(s), smooth top and tap to release extra air.
Leave on counter for at least 3 hours or overnight to form.
Generously sprinkle working surface with powdered sugar, turn out marshmallow mixture, generously sprinkle with powdered sugar, and cut as desired, lightly coating marshmallow shapes with powdered sugar to avoid sticking.
If desired, dip the marshmallows first in melted chocolate and then into toasted coconut. Let harden in the fridge for a few hours or overnight.