In a mixing bowl, whisk together flour, cocoa powder, and a pinch of salt. Cream butter with powdered sugar until light and fluffy. Add egg yolk and vanilla, mixing until incorporated. Gradually combine dry and wet ingredients to form a dough. Chill for 30 minutes.

Preheat oven to 350°F (175°C). Roll dough to ¼ inch thickness, press into a tart pan, trim excess. Blind bake for 25 minutes with pie weights.

Pulse pistachios and sugar in a food processor until finely ground. Add hot water to blend into a smooth paste.

Heat cream in a saucepan until simmering, pour over chopped dark chocolate and let sit for 5 minutes. Stir until smooth, then mix in pistachio paste.

Pour ganache into the cooled tart shell, smooth the top and refrigerate for at least 2 hours to set.

Garnish with crushed pistachios or cocoa powder. Slice with a warm knife and serve chilled.