Instructions

1. Place 1/3 cup cornstarch and 1/3 cup powdered sugar in a small bowl and whisk to combine.
2. Lightly coat an 8×8-inch square baking pan with cooking spray. Use a paper towel to wipe the pan and make sure there’s a thin film on every surface, corner, and side. Place about 1/4 cup of the powdered sugar mixture in a fine-mesh strainer, then use it to generously coat the sides and bottom of the pan. Flip the pan over and tap out the excess.
3. Shake 1 (about 15-ounce) can chickpeas before opening. Pour off 1/2 cup of the canning liquid (known as aquafaba) into the bowl of a stand mixer (or large bowl if using an electric hand mixer). Save the remaining chickpeas and liquid for another use.
4. Place 1 cup water and 2 teaspoons agar-agar powder in a medium saucepan. Bring to a boil over high heat, whisking continuously until the powder is dissolved. Reduce the heat to medium and simmer, stirring occasionally, until very slightly thickened, about 3 minutes. Add 2 cups granulated sugar and whisk vigorously until dissolved with no lumps.
5. Increase the heat to medium high and let return to a boil without stirring. Reduce the heat to medium. Once large bubbles start to form at the edges and the center starts to froth, boil for 5 minutes (the temperature will be 225℉ to 230℉). Meanwhile, add 1/4 teaspoon cream of tartar, 1/4 teaspoon xanthan gum, and 1/8 teaspoon kosher salt to the aquafaba (if you are adding food coloring, add a few drops at this stage to reach the desired color). Beat with the whisk attachment, start beating on low speed and gradually increasing to high speed, until stiff peaks form, and it looks similar to shaving cream, about 3 minutes total. Leave in the mixer.
6. When the syrup is ready, remove the saucepan from the heat. Turn the stand mixer on to low speed. Very gradually drizzle the syrup down the sides of the bowl while slowly increasing the speed to medium-high (about 1 1/2 minutes), then continue to beat until glossy, fluffy, and voluminuous, about 5 minutes total from when you started adding the syrup. Scrape down the sides with a rubber spatula as needed. Add 4 teaspoons vanilla extract and beat until combined, about 15 seconds.
7. Quickly transfer the mixture to the baking pan. Spread evenly with an offset spatula or a bench scraper, making sure you get into the corners (as you spread, you may see tiny bumps, but this is trapped air). Firmly tap the pan a couple of times on the counter to disperse some of the air in the mixture; it can be common to get some tiny bubbles in the center of vegan marshmallows, so tapping the pan will help.
8. Sift 1 to 2 tablespoons of the powdered sugar mixture through a fine-mesh strainer over the marshamallows to lightly cover. Let the marshmallow slab sit uncovered at room temperature until fully set inside, about 1 hour.
9. Run an offset spatula or knife around the edges of the pan. Use a large, flat spatula to carefully lift the marshmallow slab out of the pan to loosen it. Flip the pan onto a cutting board so the slab is powdered sugar-side down. Sift another 1 to 2 tablespoons of the powdered sugar mixture over the marshamallow slab. Transfer the remaining powder to a large shallow bowl or plate.
10. Using a sharp knife or pizza wheel, cut the marshmallows into 25 (1 1/2-inch) squares, or cut to the size of your choice. Toss the marshmallows generously in the remaining powdered sugar mixture to coat all the sides. Sift the remaining powder mixture into airtight containers before stacking the marshmallows for storage. Use parchment paper to between the layers, dusting each layer as you stack.