Prepare: Soak the shelled pistachios in a bowl of water for 4 hours, or quick-soak them in boiled water for 30 minutes to an hour). Rinse the pistachios with clean water and remove as much of the skins as possible using your fingers. They should come off easily if soaked properly.Line the base of a 9" tart tin with a piece of parchment paper and set aside.
Make the base: Add the walnuts, cacao powder, and salt to a food processor and blitz until they form a fine crumb consistency. Add the maple syrup and blend again to combine. The mixture should stick between your fingers like dough. If the mixture doesn't stick, add a little more maple syrup or water (1 tablespoon at a time) and blend again until it does.Press the dough into the bottom and sides of the tart tin using your fingers first, and then use a spoon or spatula to compact it down really well. Place in the freezer for 30 minutes until firm.
Make the pistachio paste: Place the peeled soaked pistachios, maple syrup, vanilla, and matcha in a high-speed blender or food processor and blitz for 5-10 minutes until really smooth. Scrape down the sides of the jug every minute or so in between blends. Check that the tart base is solid. Then spread the pistachio paste on top of the crust using an offset spatula to smooth it out in an even layer. Place in the fridge while you make the ganache.
Make the ganache: Add the coconut milk to a saucepan and heat up to a simmer, but do not boil.Add the finely chopped chocolate to a heat-resistant bowl and pour the heated coconut milk over it. Allow it to sit uncovered for 5 minutes, then stir until smooth. If the chocolate hasn't melted through, place the bowl on a double boiler to heat it through. Whisk in the maple syrup and salt. Pour the ganache into the tart shell. Gently tap the tart on the counter to knock out any air bubbles and smooth out the top with a spatula if needed. Chill for a minimum of 4 hours, or overnight.
Garnish the tart: Carefully remove the tart from the tin and place it on a serving plate. Sprinkle chopped pistachios around the outer edges.
Storage: Refrigerate in a sealed container, or on a plate covered with plastic wrap for up to 5 days.Freeze for up to 3 months, and defrost in the fridge the night before serving.
Chilled coconut milk: Refrigerate the cans of coconut milk overnight, then only use the thick cream from the top of the cans. You will need 2x 400ml cans for this recipe.
You can use Pistachio cream (crema al pistachio) instead of homemade paste, just make sure it does not contain any added dairy or honey.
Pistachio butter is also fine to use, but it usually has a runner consistency so be prepared for the filling to ooze out.
Make sure the crust is firmbefore adding the pistachio paste to ensure you get nice clean layers.