Preheat oven to 180°C bake (160°C fan-forced). Grease and line the base of an 18cm x 28cm tin with baking paper.
Melt the butter and chocolate in a large bowl set over a saucepan of simmering water, stirring until smooth and glossy. Add the Chelsea Soft Brown Sugar or Chelsea Dark Cane Sugar, cocoa and vanilla and stir to combine.
Add the eggs one at a time, beating well after each addition. Mix in the flour, pecans (optional) and salt until combined. Pour into the prepared tin and bake for 20-25 minutes (the centre should be quite fudgy and dense and should stick to a testing skewer).
Cool in the tin then cut into bite size pieces.
Dust with Chelsea Icing Sugar before serving.