Set the oven temperature to 180°C.
Melt the butter in a saucepan and add the chopped chocolate. Stir into a smooth paste and set aside.
Whisk together the eggs and the two sugars until thoroughly mixed. Add the butter mixture, orange zest and orange juice.
Combine the flour and salt, sieve them into the mixture and stir together. Pour the mixture into a greased springform tin measuring 22-24 cm and bake for about 20 minutes. The cake should be slightly runny inside. Leave the cake to cool completely.
Peel and segment the oranges. Decorate the cake with orange fillets and sprinkle with chopped pistachios.
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