Melt the chocolate in a bowl placed over a pan of hot, but not boiling, water. As soon as the chocolate has melted, remove the bowl from the heat and leave to cool a little.
Grind the pistachios with 50g of the granulated sugar in a coffee grinder or blender until finely ground.
Cream the butter with another 50g of sugar until very pale. Beat in the ground pistachios.
Separate the eggs. Beat the egg yolks into the cream and butter mixture one by one. Stir in the melted chocolate. Don't wash the bowl, you will need it again.
Whisk the egg whites with a pinch of salt in a separate bowl until stiff. This is made easier if you have electric beaters or a food mixer. Whiske in the remaining 50g of granulated sugar. Fold this into the cake mixture one third at a time.
Spoon mixture into a 23 cm round silicone cake mould or lined tin. Bake for 15 minutes at 190℃ (375℉, Gas 5). Turn the oven down to 180℃ (350℉, Gas 4) and bake for a further 20-25 minutes. Turn out onto a wire rack to cool.
Melt the dark chocolate in a bowl over hot water with the cream and fruit liqueur (I used homemade sea buckthorn liqueur). Stir together lightly with a small whisk to avoid the mixture splitting.
Spread on top of the cooled cake. Scatter with a few chopped pistachios.
Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.