Start by steaming the spinach in a pot or saucepan with a bit of water in the bottom. Bring to a boil and steam until the spinach is just wilted, no more. You want it to retain its bright green color so don’t overcook it. Drain and squeeze out the water, then chop it a bit.
Combine the spinach with the soaked and drained cashew nuts, coconut milk, lemon juice, palm sugar, 2 tablespoons matcha powder, and the vanilla paste in a blender and blend until everything is completely smooth. If your coconut milk is very thick then add about a 1/4 cup of almond milk as well, this will make it easier to get the mixture completely smooth.
Pour the almond milk into a small pot and whisk in the agar-agar powder. Heat the milk over low heat and let it simmer for about 30-45 seconds, whisking continuously. Be careful it doesn’t boil over, it can happen quickly.
Start up the blender with the green filling mixture and, with the blender running, pour in the agar-agar milk and blend everything until it is all nicely mixed, then quickly pour the filling over your almond/coconut base. Gently tap the spring form a few times against your worktop to get rid of whatever air pockets or bubbles may be hiding in the filling.
Put your cake in the fridge until nicely set, about an hour.
Before serving, remove it from the spring-form and dust the top with extra matcha powder (using a small sieve) and decorate with fresh berries.