Prepare a 9-inch pan by lining it with parchment paper and lightly dusting it with a mixture of powdered sugar and cornstarch. This will prevent the marshmallow from sticking. You'll want to do this first since you want to transfer your marshmallows immediately after you're finished mixing, else it'll begin to firm up in your mixing bowl and won't be that smooth delicate pour.

In the bowl of a stand mixer fitted with a whisk attachment, pour in the aquafaba and add the cream of tartar. Start whisking at medium speed until the aquafaba forms soft peaks. (This should take about 10 minutes on medium speed or medium-high speed if you're not using chilled aquafaba.)

In a saucepan, combine 1 cup of granulated sugar and ⅓ cup of water. Place the saucepan over medium-high heat and let the mixture come to a simmer without stirring. Insert a candy thermometer into the syrup and continue simmering until it reaches 240°F (115°C). It should take approximately 10 minutes.

Once the sugar syrup reaches the desired temperature, slowly drizzle it into the whipped aquafaba while continuing to whisk. Keep whisking for an additional 5-8 minutes until the aquafaba becomes glossy and stiff peaks form. Add the vanilla extract during this process.

In a saucepan, combine ¼ cup of granulated sugar, ¾ cup of water, and the agar-agar powder. Mix the ingredients thoroughly. Place the saucepan over medium-high heat and bring it to a boil. Whisk continuously for 2-3 minutes while the mixture simmers.

Slowly pour the agar mixture into the whipped aquafaba, whisking constantly until the mixture is fully incorporated. Continue to whisk until the agar is incorporated into the aquafaba mixture, which shouldn't take more than 1 minute.

Using a spatula, transfer the marshmallow mixture to the prepared pan, spreading it evenly. You won't get everything out, closer to about 90% out, and don't try to get it all out because it'll start to set as it hits the pan and you want to focus on smoothing that part down. If you're having difficulty spreading the marshmallow fluff into an even texture, slightly wet your hands of the spatula to help you pat it down. Loosely cover the pan with a towel, allowing the marshmallow to set at room temperature for 4-6 hours.

Once the marshmallow has set, dust the top with the remaining powdered sugar and cornstarch mixture, ensuring a light coating. Loosen up the edges with a dinner knife, then dust a sharp knife with your cornstarch powdered sugar mixture.

Cut out 36 square marshmallows (6 rows, 6 columns) or carefully cut the marshmallow into desired shapes. If desired, roll the cut marshmallows in the sugar-cornstarch mixture, ensuring all sides are coated. Gently tap off any excess mixture. Enjoy!