- With the rack in the middle position, preheat the oven to 180 °C (350 °F). Line the bottom of a 20-cm (8-inch) square baking pan with a strip of parchment paper, letting it hang over two sides. Butter the other two sides.
- In a bowl, over a double boiler or in the microwave oven, melt chocolate and butter. Add oil. Set aside.
- In another bowl, combine cheese, brown sugar and salt with an electric mixer until smooth.
- Add eggs, one at a time, and beat until mixture is smooth. On low speed, add chocolate mixture and flour. If desired, stir in pistachios with a spatula.
- Spoon into the prepared pan and bake for about 40 minutes or until a toothpick inserted in the centre comes out with some lumps and not completely clean.
- Cool completely in the pan. Unmould and cut into squares. Serve at room temperature.