With the rack in the middle position, preheat the oven to 165 °C (325 °F). Line the bottom of a 20-cm (8-inch) square baking dish with a strip of parchment paper, letting it hang over each side. Butter the other two sides.
In a bowl over a double boiler or in the microwave oven, melt the chocolate with the butter. Let cool.
In another bowl, combine the eggs, sugar, and salt with a whisk until smooth. Add the chocolate mixture and flour and stir until smooth.
Pour into the prepared pan. Bake for about 35 minutes or until a toothpick inserted in the centre of the cake comes out with a few lumps and not completely clean.
Let cool in the pan for 2 hours. Unmould and cut into squares.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.