Preheat your oven to 350°F (175°C) and prepare a 9-inch springform pan by lining the bottom with parchment paper and greasing the sides.

Place ground pistachios in a food processor and pulse for 2 to 3 minutes until reaching a fine, sandy texture.

In a large bowl, whisk together egg yolks and ½ cup granulated sugar until pale and creamy. Incorporate melted butter, vanilla extract, and lemon zest.

Gently fold ground pistachios and a pinch of salt into the egg yolk mixture to combine without overmixing.

In a separate bowl, beat egg whites until stiff peaks form, gradually adding the remaining sugar.

Carefully fold whipped egg whites into the pistachio mixture, starting with one-third to lighten the batter.

Pour the batter into the springform pan and smooth the top. Bake for 40 to 45 minutes, or until a toothpick comes out with moist crumbs.

Allow the cake to cool for 10 minutes in the pan before removing and prepare the chocolate ganache by heating heavy cream and mixing with dark chocolate until smooth.

Add a pinch of salt and melted butter to the ganache, then cool slightly before pouring over the cooled cake.