Sift the matcha powder into a small bowl. Use a double boiler to heat or microwave the whipping cream to about 70 to 75°C. Pour the heated whipping cream to the matcha powder, combine them with a tea whisk or a mini egg whisk. Let cool and set aside.
Separate the egg whites and yolks. Make sure the mixing bowl for the egg whites are clean and dry.
Beat the egg yolks, and add the matcha cream made on step 1. Mix well. Then sift in the okara powder, add the pink salt and vanilla extract (if using). Whisk until no visible lumps.
Beat the egg whites with an electric/stand mixer at low speed. When the egg whites turn foamy, start to add in monk fruit sweetener in three batches, and slowly beat the egg whites until stiff peaks form (about 18-20 minutes). Then add apple cider vinegar to the meringue and mix for around 30 seconds more. The acid can help stabilize the meringue.
Preheat the oven to 150°C.
Use an electric/stand mixer to beat the egg yolk batter made on step 3 at low speed until thick and more pale in color, around 30 to 45 seconds.
Add 1/3 of the meringue to the egg yolk batter, mix gently with a hand mixer, then pour the mixture into the remaining meringue. Mix them lightly with a hand whisk or a silicone spatula until they’re thoroughly combined.
Do not grease the cake pans. If you aren’t using removable bottom baking pans, line the bottom of the pans with parchment paper. Pour the mixture evenly into two cake pans (diameter of 4” and height of 2.5”). They should be 50-60% full.
Bake in the preheated oven at 150°C for 45 to 48 minutes*see Note 3. Insert a bamboo skewer into the center of the cake to check the doneness. If there is no cake batter sticking on the skewer, take the cakes out and immediately turn them upside down to let cool completely. You may use two mugs with the same height to support the edge of the pan while putting the cake upside down, or use a cooling rack specifically for chiffon cakes.