Lightly butter an 8-inch x 8-inch baking pan and line with parchment paper, buttering the parchment as well.

In a stand mixer bowl, pour in 1/2 cup of cold water and the vanilla extract (if using). Stir in the gelatin powder and let it bloom without stirring.

In a medium saucepan, combine the remaining 1/2 cup of water, granulated sugar, light corn syrup, and kosher salt. Cook over medium-low heat, stirring occasionally until sugar dissolves.

Increase heat to medium-high and cook until the thermometer reads 250°F (about 5 minutes). Remove from heat and let cool to 220°F (approximately 5 minutes).

With the mixer on low, slowly pour in the hot sugar syrup to melt the gelatin. Increase speed to medium-high and whip until fluffy, tripled in volume, and thick—about 10 minutes.

Pour the fluffy mixture into the prepared pan, smooth into an even layer, cover loosely with foil, and let set for 2 hours in the refrigerator or 8 hours at room temperature.

Whisk together powdered sugar and cornstarch in a medium bowl. Dust the top of marshmallows and a cutting board.

Invert marshmallows onto the board, peel off parchment, and cut into 1-inch squares, dusting all cut edges.

Line a baking sheet with parchment. Temper chocolate as preferred. Use a chocolate dipping fork to fully submerge each marshmallow in melted chocolate.

Lift fork and tap 3-5 times to let excess chocolate drip off, then slide chocolate-covered marshmallow onto the prepared baking sheet.

Repeat for remaining marshmallows and store in a cool, dry place.