Preheat the oven to 350F (180C). Grease with butter a 8 or 9-inch (20-22cm) pan and line with parchment paper.
Melt the chocolate and butter over a bain-marie until smooth. Let cool to room temperature.
Meanwhile, add the eggs with sugar, salt and vanilla extract into a large bowl.
Mix for few minutes until thick and light yellow.
Add melted chocolate.
Mix until well combined.
Sift the flour.
Using a spatula, fold in sifted flour.
Pour the mixture into the prepared pan.
Spread the batter evenly into the pan.
Bake for about 25 minutes until set. Run a sharp knife completely around the inside edge of the pan.
Let cool for 10 minutes and invert onto a serving plate. Let cool completely.
Chop pistachios for the crunchy layer.
Spread ¼ cup of pistachio cream over the cake.
Sprinkle chopped pistachios on top of the cream, and press them gently to stick.
Refrigerate until you prepare the cream cheese pistachio mousse.
Dissolve gelatin in cold water and let it swell for about 5 to 10 minutes.
Mix the cream cheese with powdered sugar.
Add pistachio cream and matcha powder if used.
Dissolve gelatin over low heat and pour over the pistachio mixture.
Mix to combine.
Add the cream and start mixing until soft peaks form.