- 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.