Preheat oven to 175°C. Line a muffin tin or a baking dish (measuring about 28x22cm) with baking paper.

Melt butter in a small pot on a low heat then remove from heat and add chocolate. Stir gently, until melted then transfer to a large bowl. Add sugar to bowl and mix to combine.

Add eggs, one at a time, mixing after each addition.

Fold in flour, cocoa powder and salt until just combined. Be careful not to over-mix.

Divide mixture between muffin tins or pour into prepared dish, spreading into an even layer.

Bake for 25-30 minutes, until a knife inserted into the centre comes out crumby but not wet. Allow to cool for 10 minutes before serving.