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 dish with a strip of parchment paper, letting paper hang over two sides. Butter the other two sides.
In a bowl, over a double boiler or in the microwave oven, melt chocolate with butter. Let cool.
In another bowl, using an electric mixer, beat eggs with brown sugar and salt until smooth. At low speed, add chocolate mixture and flour and beat until smooth. With a spatula, fold in half of the nuts and marshmallows.
Spoon batter into the prepared pan. Bake for about 35 minutes or until a toothpick inserted in the centre of the cake comes out with lumps and not completely clean.
Right out of the oven, sprinkle with remaining nuts and marshmallows. Let cool.
Meanwhile, in a saucepan, bring sugar and water to a boil. Cook without stirring until mixture turns golden brown. Remove the pan from the heat and add cream. Bring to a boil, stirring until smooth. Cool slightly and drizzle over cooled cake. Serve warm or 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.