Combine 1/2 cup cold water and the gelatin in the bowl of a stand mixer fitted with the whisk attachment. Mix lightly to incorporate.
In a small metal or ceramic bowl (preferably not plastic), whip the egg whites until stiff peaks form. Set aside.
In a small saucepan, combine 2 cups of the granulated sugar, the simple syrup, 1/2 cup room temperature water, and salt. Heat over medium-high heat until the mixture reaches 240 degrees F on a candy thermometer, 7 to 8 minutes. Immediately remove from the heat.
With the stand mixer on low speed, slowly pour the sugar syrup down the side of the bowl and into the gelatin mixture. Increase the speed to medium and mix until the gelatin has dissolved. Turn off the mixer and add the whipped egg whites. Turn the mixer to high and whip until the mixture becomes lukewarm and very thick, 12 to 15 minutes. Add 1/2 teaspoon of the cinnamon during the last minute of whipping.
While the mixture is whipping, prepare the pan. Combine some confectioners' sugar and tapioca flour in a small bowl. Lightly spray a 9-by-9-inch metal baking pan with cooking spray. Add the confectioners' sugar and tapioca mixture and coat the bottom and sides of the pan.
Combine the remaining 1/4 cup cinnamon and the remaining 1/2 cup granulated sugar in a small bowl. Lightly coat the bottom of the pan with cinnamon sugar; reserve the remaining mixture.
Pour the marshmallow mixture into the prepared pan and use a lightly oiled spatula to spread it evenly. Dust the top with enough of the remaining cinnamon sugar to lightly cover; reserve the rest. Let the marshmallows sit, uncovered, for at least 6 hours and up to overnight.
Turn the marshmallows out onto a cutting board and cut into 1 1/2-inch squares using a pizza wheel sprayed with cooking spray. Lightly dust all sides of each marshmallow with the remaining cinnamon sugar. Store in an airtight container for up to 3 weeks.
NotesThese marshmallows can also be cut smaller for hot cocoa and coffee.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.