Grind black sesame and soy milk together in a blender.
Separate the egg whites and yolks.
Add low-calorie sweetener to the egg whites and whip until foamy.
Mix the black sesame-soy milk mixture into the egg yolks.
Combine the whipped egg whites with the yolk batter.
Sift and fold in the low-carb flour mixture.
Pour the batter into a castella mold.
Bake at 180°C (356°F) for 25 minutes to finish.