1. Preheat the oven to 170°C fan or 180°C with top and bottom heat. Sift the cocoa into a bowl. Heat 100 ml water and remove just before it boils. Pour into the cocoa in a thin stream, stirring vigorously.

2. Put the melted butter in another bowl; it must not be hot. Add the sugar and stir to combine. Then add the eggs one at a time, stirring after each.

3. Sift the flour and baking soda over the mixture and add the vanilla. Stir, add the cocoa mixture and stir again.

4. Once smooth, add the chocolate chips or chopped chocolate and mix thoroughly.

5. Do not use a mixer or whisk: use a wooden spoon or baking spatula. Line a 20–22 cm square tin with baking paper, or grease with butter and dust with flour.

6. Pour in the batter. Bake for 25 minutes at 170°C fan, or for 20 minutes at 180°C with top and bottom heat.

7. Leave to cool, cut into squares and serve.