Important tip: Anytime you are not using the tempura batter make sure to keep it in the fridge. It needs to be ice cold to work out the best.
To make the tempura, add aquafaba to a medium bowl and vigorously whisk until foamy. Add the soda water while continually whisking to keep the foam. Add the flour, salt, brown sugar and baking powder and whisk until smooth. Add the oil and mix until smooth - if the batter is thick add another splash of soda water to thin it out. It should be like a crepe batter. Store in the fridge until ready to use.
In a dutch oven or a deep pot pour in the vegetable oil until it's about 2" deep. Heat until 350F.
Add the chopped vegetables to the batter and coat. Place individually in the hot oil and fry until golden brown or about 3-4 minutes. Remember not to over-crowd the oil as this is a serious safety hazard. Check out our deep frying safety tips for more information.
Place fried tempura on a plate with paper towel to absorb any extra grease. Serve with soy sauce or spicy mayo.