Preheat the oven to 160°c / gas 3. Line a 8in (20.5cm square tin with baking paper and set aside.
Gently melt the chocolate in a heatproof bowl over a pan of simmering water, with the butter and 2 tbsp water. Remove the bowl from the pan and cool slightly.
In the meantime, in a mixing bowl add the sugar and eggs and whisk together light and fluffy, then add the vanilla.
Pour the melted chocolate into the egg mixture and combine together.
Gently fold in the flour and a pinch of salt until all mixed together.
Pour into the tin and back in the oven for 30-35mins until risen and just firm. The brownie should be gooey in the centre.
Leave to cool in the tin for 10mins, then cut into squares with a sharp knife. Top tip: when cutting into squares, if you wipe the blade after each cut you will get nice clean cut squares.