In a medium bowl, whisk together the all-purpose flour, cocoa powder, and salt.

In a separate bowl, cream the softened butter and powdered sugar until light and fluffy, about 2-3 minutes.

Mix in the egg yolk and vanilla extract, then gradually incorporate the dry ingredients to form a cohesive dough.

Wrap the dough in plastic wrap and chill in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C). Roll out the dough to fit your tart pan.

Transfer the dough into the pan, cover with parchment, and fill with pie weights.

Bake for 15 minutes, then remove the weights and bake for another 10 minutes until set.

Let the crust cool completely.

In a food processor, pulse the pistachios and sugar until finely ground. Add hot water to make a smooth paste.

In a saucepan, heat the heavy cream until simmering, then pour over chopped dark chocolate.

Let sit for 3-5 minutes, then stir until smooth. Blend in the pistachio paste, butter, and vanilla extract.

Pour the ganache into the cooled tart shell and refrigerate for at least 2 hours.

Garnish with toppings and slice with a warm knife.