- 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 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, beat eggs and sugar with an electric mixer until it whitens and doubles in volume. At low speed, add chocolate mixture and flour and beat until smooth. Stir in walnuts with a spatula.
- Pour into the prepared pan. Bake for 45 to 50 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.
- Meanwhile, place chocolate in a bowl and add boiling water. Let stand for 1 minute, without stirring.
- With a whisk, stir until ganache is smooth. Add butter and stir until completely melted. Allow to cool for about 1 hour.
- Whip ganache with an electric mixer until creamy. Unmould brownies and cover with frosting.