Preheat your oven to 350°F (175°C). In a mixing bowl, combine cocoa powder, flour, and sugar. Cut in cold butter until the mixture resembles coarse crumbs. Gradually add ice water, mixing until a dough forms. Roll it out between parchment paper, fit it into a tart pan, and prick the bottom with a fork. Bake for 20-25 minutes, until fragrant and slightly firm. Cool completely before filling.

While the crust cools, gather a heatproof bowl and place chopped dark chocolate inside. In a small pot, heat heavy cream over medium heat until just simmering, then pour it over the chocolate. Let it sit for a minute to soften, then stir until the mixture is smooth and glossy. Add a splash of vanilla extract and a pinch of salt. Set aside at room temperature to cool and thicken.

In a blender or food processor, grind unsalted pistachios until finely ground. In a saucepan, combine the ground pistachios, milk, sugar, and cornstarch. Cook over medium heat, whisking constantly until the mixture thickens and bubbles gently; this will take about 5-7 minutes. Remove from heat and incorporate vanilla if desired, stirring until well combined. Allow it to cool slightly before pouring over the ganache.

Pour the smooth pistachio cream evenly over the cooled dark chocolate ganache. Spread it gently with a spatula for an even layer. Cover the tart with plastic wrap and chill in the refrigerator for 2-3 hours, or until the pistachio cream is set and firm to the touch.

Once the pistachio cream layer is firm, prepare the top ganache. In a saucepan, heat the remaining cream until simmering, then pour it over chopped dark chocolate and add butter. Whisk until the mixture is silky and smooth. Carefully pour this ganache over the set pistachio cream, letting it flow evenly across the top. Chill again for at least 1 hour, allowing it to set perfectly.

Refrigerate the complete Chocolate Pistachio Tart until the ganache is fully set, about 2 hours. Before slicing, let it sit at room temperature for 15-30 minutes for easier serving.