In the bowl of a stand mixer fitted with a whisk attachment, sprinkle gelatin over 1/2 cup of the water.
In a small saucepan, whisk together remaining 1/2 cup water, granulated sugar, brown sugar, corn syrup, and salt over medium high heat. Cover and cook for 3-4 minutes. Remove the cover and heat the mixture until it reaches 240 degrees F. Remove from heat.
While the mixer is whisking over low speed, gradually pour the hot syrup down the side of the bowl. After the syrup has been incorporated, increase speed to high and whisk until thickened, 12-15 minutes. During the last minute, slowly pour in vanilla and chocolate chips.
Grease a 9x13 inch baking sheet. In a small bowl, mix together powdered sugar and cornstarch. Pour into the greased baking sheet, shaking it around to fully coat. Pour the excess back into the bowl.
Pour the thickened marshmallow into the pan. Grease a spatula and even out the mixture as much as possible. Sprinkle the top with remaining sugar and cornstarch mixture, spreading to thoroughly coat. Remove excess back to the bowl. Let the marshmallows rest uncovered at room temperature 4 hours to overnight.
Flip the pan over onto work surface. Use a pizza cutter or large knife dusted with powdered sugar to cut the marshmallows into 1 inch squares. Dust the cut sides of the marshmallows with remaining powdered sugar and cornstarch.
Store in airtight container and consume within 3 weeks.