Dust two 8x8-inch pans with powdered sugar. Set aside.

Add aquafaba and cream of tartar to the bowl of a stand mixer. Mix with the whisk attachment on high for 6 minutes, until fluffy. Add vanilla and whip another 3 minutes until stiff peaks form.

In a small saucepan, combine water and agar agar, and bring to a boil. Boil the water for 3 minutes, stirring continuously.

Add sugar and boil over medium low heat for another 3 minutes. (Be mindful—don’t let the mixture burn!)

Turn off the heat. Add salt to aquafaba mixture and gradually pour in hot syrup, beating on low for 30 seconds at a time (for about 2 minutes). The mixture will turn white and shiny.

Add mixture to prepared pans, spreading into an even layer.

Dust the tops with powdered sugar and allow to cool for 1-2 hours.

Preheat oven to 150°F (65°C), or lowest possible temperature.

Cut marshmallow mixture into squares and transfer to baking sheets lined with parchment. Dust marshmallow squares with powdered sugar on all sides.

Turn the oven off and add the cubed marshmallows to warm oven for about an hour to help them dry out.