Heat oven to 180C/160C fan/gas 4. Butter a 30cm x 20cm non-stick tin with butter and line the base with non-stick baking parchment.

Fill a small saucepan a third full with water, bring to a simmer and put a snug-fitting heatproof bowl on top of it. Add the butter and chocolate and gently melt over a low heat, stirring occasionally. Be careful not to let it catch and burn on the bottom. Remove from the heat and leave to cool a little.

Beat the eggs and sugar together using an electric whisk for 8-10 mins or until thick enough to leave a trail. Gently fold through the cooled melted chocolate and vanilla, followed by the flour, cocoa and salt. Finally, fold through the chocolate chunks. Pour the brownie batter into the lined tin, place in the centre of the oven and bake for 30-35 mins.

Leave to cool a little in the tin before cutting into 12 squares.