- With the rack in the middle position, preheat the oven to 170°C (325°F). Line the bottom of a 20-cm (8-inch) square baking pan with parchment paper and butter the sides.
- In a bowl, combine the flour, cocoa powder and salt. Set aside.
- Improvise a double boiler by setting a heavy glass or ceramic mixing bowl on a saucepan partially filled with simmering water (the bottom of the bowl should be over, not in, the water). Melt the chocolate and butter in the bowl. Remove the bowl from the saucepan and whisk in the sugar. Add the vanilla. Add the eggs one at a time and whisk for 2 minutes. Gently add the dry ingredients. Spread the batter evenly in the baking pan. Bake until the centre is set but still moist, 30 to 35 minutes.
- Let cool in the pan, 2 to 3 hours.