Pour the aquafaba in a mixing bowl and add the cream of tartar.Mix on high speed for 4 minutes until soft peaks form.
Gradually add the icing sugar (1 or 2 tbsp at a time) and beat until glossy peaks form.
Add the vanilla powder and the soy milk. Beat until you obtain a fluff texture (like melted marshmallow).
Mix together all the dry ingredients in a large bowl.
Pour the wet ingredients on the dried one and stir with a spatula.
Once you've obtained a dough, add the softened vegan butter and incorporate it by kneading the dough with the spatula.
Pour the batter into a pipping bag and place it to the refrigerator for 30 minutes.
Preheat your oven at 175°C.
Pipe the batter on a baking sheet: 1/1.5 cm wide and 8/9 cm long. You should be able to pipe around 30 Savoiardi.Bake for 13 minutes at 175°C.
Notes
[1] This is the liquid that is present in the canned chickpeas.[2] Can be replace by vanilla extract.[3] Can be replaced by cake flour or plain flour + 1 tbsp of cornflour + 1/2 tsp of baking powder