Make the pistachio butter (this can be done up to a week ahead of time): preheat the oven to 350 F. Spread the (240g) pistachios on a baking tray in an even layer. Bake until hot and fragrant, for about 9 minutes, shaking the pan halfway for even heating. Transfer the nuts to a food processor and process on high until you have pistachio butter. This can take up to 10 minutes, depending on the strength of your machine. The nuts will first turn finely ground, keep processing and scraping down, until the nuts turn into a loose paste. Let cool, store in an airtight container until ready to use.

Make the crust: preheat the oven to 350 F. In a bowl, whisk together the cocoa (40g or ½ cup dutch process cocoa powder), flour (150g or 1 cup plus 2 tablespoons), powdered sugar (65g or ½ cup powdered sugar), and salt. Add the softened butter (120g or 9 tablespoons butter) and use a hand mixer to beat until the butter is in tiny bits. Add the egg yolk (or 2-3 tablespoons milk) and beat until the dough starts to clump together. Transfer the dough to a 9 inch tart pan (it should be about an inch tall) and press the dough into the bottom and up the sides of the pan. Take your time with it, trying to get it in an even layer. Use the bottom of a measuring up (flour it to keep it from sticking) to pack the crust in. Lay a sheet of parchment paper over the dough and fill it with pie weights. Bake the crust for 15 minutes then remove the parchment and pie weights, dock the crust all over with a fork (to prevent bubbling) and return to the oven to bake until the crust is set, about another 12 minutes. Let cool completely before moving onto the ganache.

Make the ganache: chop up the chocolate (180g) very finely. Set the heavy cream (200g) in a pot over medium low heat and warm until just the sides start bubbling. (I like to give my chocolate a gentle warm, 30 seconds in the microwave, before adding the hot heavy cream). Pour the heavy cream over the chocolate and gently stir, then leave it for about 30 seconds. Stir, adding the salt and vanilla as you do, until the ganache is smooth. Pour the ganache into the cooled tart crust. Smooth into an even layer and set in the fridge for at least an hour to firm up.

Make the pistachio mousse: set the room temperature cream cheese (226g or 1 brick) in a large bowl and use a wooden spoon or rubber spatula to press it into the bowl (this helps remove any lumps). Add the vanilla and salt and use a hand mixer (you can also do this in a stand mixer with the paddle attachment) to beat it until smooth. Add the pistachio butter and beat until combined. Add the powdered sugar (65g or ½ cup) and heavy cream (360g or 1 ½ cups) (if you are using the stand mixer switch to the whisk attachment) and beat until stiff peaks; the whisk/beaters should leave ripples that hold shape. Spread the pistachio mousse on the pie and smooth into an even layer. Top with pomegranate arils and/or strawberries or raspberries. Chill in the fridge for about 2-6 hours until the mousse is set. Serve chilled and store in the fridge (it will keep for 2-3 days).