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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.