Combine 1/2 cup water, salt and gelatin in large bowl of a stand mixer. Stir and set aside.
Combine remaining 1/2 cup water, sugar and corn syrup in a large saucepan over medium heat. Bring to a boil, stirring frequently. Using a candy thermometer, cook until mixture reaches 240 degrees F or soft ball stage.
Gradually pour the hot syrup over the gelatin mixture with the mixer on low. Turn the mixer on its highest speed and beat for 8 minutes. Add vanilla and beat an additional 2 minutes. The mixing bowl will get very full. The marshmallows will become very thick, white and shiny. Spread mixture onto greased baking sheet with sides. For thicker marshmallows, use a 13 x 9-inch pan.
Let marshmallows cool at least 4 hours or overnight. Combine powdered sugar and corn starch in a shallow pan. Cut the marshmallows into bite-size pieces with a sharp knife or scissors, dropping one at a time into the powdered sugar. Toss to coat well. Store marshmallows in an airtight container in a cool place. Best used within 1 month.