Combine the powdered sugar and cornstarch. Dust the prepared pan generously with some of the mixture and reserve the rest for coating the marshmallows.

Pour 1/2 cup cold water into a small bowl. Sprinkle the gelatin evenly over the water and let it bloom for 10 minutes until thickened.

Add the granulated sugar, light corn syrup, remaining 1/2 cup water, and salt to a medium saucepan.

Heat gently until the sugar dissolves. Once the mixture begins boiling, stop stirring and continue cooking until the syrup reaches 240°F (116°C).

When the syrup reaches about 225°F, begin whipping the pasteurized egg whites on medium speed until soft peaks form.

Remove the sugar syrup from the heat once it reaches 240°F. Carefully stir in the bloomed gelatin until completely dissolved.

Turn the mixer to low speed and slowly pour the hot syrup down the inside edge of the mixing bowl while the egg whites continue mixing.

Increase the mixer speed to medium-high and whip for 8–10 minutes, or until the marshmallow mixture becomes thick, glossy, bright white, and greatly increased in volume.

Add the vanilla extract during the final minute of whipping and mix until evenly incorporated.

Immediately scrape the marshmallow mixture into the prepared pan and spread it evenly using a lightly oiled spatula.

Dust the top generously with the remaining powdered sugar and cornstarch mixture.

Leave the marshmallows uncovered at room temperature for 6–8 hours, or overnight, until completely set.

Lift the marshmallow slab from the pan using the parchment paper. Cut into 24 squares with a lightly oiled knife or pizza cutter.

Coat every cut side of the marshmallows with the remaining powdered sugar and cornstarch mixture before serving or storing.