Preheat the oven to 325F (160C). Grease with butter a 8 inch (20cm) pan and line with parchment paper.
Melt chocolate and butter over a bain-marie until smooth. Let cool slightly.
Meanwhile mix the eggs with sugar and salt using a whisk or mixer until thick and light yellow, for about 5 minutes. Mix in vanilla extract or your favorite extract (mint, rum, almond, pistachio etc).
Add melted chocolate and mix until well combined.
Using a spatula fold in the cocoa powder. Pour mixture into the prepared pan.
Place the pan into a larger baking tray and add hot water, up to halfway of the pan.
Place into the oven and bake for 25 minutes and remove the pan from the baking tray.
Let cool completely in the pan and refrigerate for several hours or overnight.