1. Grease an 8×8 inch pan and dust generously with a mixture of powdered sugar and cornstarch. Set aside.

2. In a small saucepan, combine granulated sugar, corn syrup, water, and salt. Heat over medium-high heat, stirring until sugar dissolves, then stop stirring and bring to 240°F (116°C).

3. While the sugar mixture is heating, whip egg whites in a stand mixer with the whisk attachment until soft peaks form.

4. Once the sugar mixture reaches 240°F, slowly pour the hot syrup into the whipping egg whites in a steady stream, being careful not to hit the whisk. Continue to whip on high speed until the mixture is very thick, glossy, and has cooled to room temperature (about 10–15 minutes).

5. Add the vanilla extract and whip for another minute until fully incorporated.

6. Scrape the marshmallow mixture into the prepared pan and spread evenly with an offset spatula. Dust the top generously with the remaining powdered sugar and cornstarch mixture.

7. Let the marshmallows set at room temperature for at least 4 hours, or preferably overnight.

8. Once set, invert the marshmallow slab onto a cutting board dusted with more powdered sugar mixture. Cut into desired shapes (squares, or even little Stay Puft shapes!) using a greased knife or cookie cutters.

9. Store in an airtight container at room temperature for up to 2 weeks, ensuring they are well-dusted to prevent sticking.