Boil the spinach, chop it into small pieces, then mix it with the soy milk in a blender. It's okay to have small lumps of spinach remaining in the mixture!
Put the egg whites in a bowl with a pinch of salt. Beat into a stiff meringue with a hand-held mixer. Refrigerate until use.
Put the egg yolks and honey in a separate bowl. Beat with the hand-held mixer until the mixture has a consistency similar to mayonnaise. Add the mixture from Step 1 and mix.
Sift the rice flour into the mixture from Step 3 and mix thoroughly. Use a hand-held mixer up until this step.
Add 1/3 of the Step 2 meringue to the bowl from Step 4. Mix well with a whisk until the air bubbles disappear.
Add the remaining meringue of Step 2 and fold it into the batter with a spatula, scooping it up from the bottom of the bowl, while rotating the bowl. Don't over-mix!
Pour the batter into a tube pan. Place your thumbs on the center tube and other fingers around the rim of the pan to hold it down securely. Lightly tap the pan on your work surface once or twice to release the trapped air.
Bake at 170C (325F) for 40 minutes. If the top browns too fast while baking, cover with aluminum foil.
The cake is baked when a skewer inserted into the cake comes out clean. Immediately invert the pan and balance it upside down on a glass jar. Then, let it cool.
When the cake has cooled to the touch, cover with a plastic bag to keep it moist, and refrigerate it upside down! Remove the cake from the mold after it cools completely.