1. Lightly grease a 9×13 inch baking pan and dust generously with a mixture of powdered sugar and cornstarch. Set aside.

2. In a medium saucepan, combine granulated sugar, corn syrup, water, and salt. Cook over medium-high heat, stirring until sugar dissolves. Continue cooking without stirring until the mixture reaches 240°F (115°C) on a candy thermometer.

3. While the sugar mixture is cooking, begin whisking the egg whites in the bowl of a stand mixer fitted with the whisk attachment until soft peaks form.

4. Once the sugar mixture reaches 240°F, remove from heat. With the mixer running on low speed, slowly and carefully pour the hot syrup in a steady stream into the egg whites. Increase speed to high and beat until the mixture is very thick, glossy, and holds stiff peaks, about 8-10 minutes. Beat in the vanilla extract.

5. Immediately scrape the marshmallow mixture into the prepared baking pan. Using an offset spatula lightly greased, spread evenly.

6. Dust the top generously with more of the powdered sugar and cornstarch mixture. Let sit at room temperature for at least 4 hours, or overnight, until set.

7. Once firm, invert the marshmallow slab onto a cutting board dusted with the powdered sugar mixture. Cut into desired shapes using a sharp, greased knife or cookie cutters. Dust all cut sides with the powdered sugar mixture to prevent sticking. Store in an airtight container at room temperature.