Preheat the oven to 180°C / 350°F / gas 4. Prepare your lined tin - I use a 20.5 x 30.5cm one. Put the butter in a heatproof bowl along with the dark chocolate. Place a small amount of boiling water into a saucepan and rest the heatproof bowl with the butter and chocolate on top, then melt the contents on a low heat. You don't want the water to boil. Once melted set aside.
Add the sugar and eggs to your electric mixer then whisk for about two minutes until your mixture is slightly frothy.
Pour the chocolate mixture into the bowl and whisk in for a few minutes until all your mixture is all combined.
Sift the flour and cocoa powder into your mixture and stir with a wooden spoon until all combined.
Add your chocolate chunk pieces and stir in.
Add your mixture to your prepared tin and pour in evenly. Gently bang the tin on the work surface to get rid of any air bubbles.
Carefully place them in the oven for 20 minutes. To test the brownies are cooked put a metal skewer in and it should come out clean.
Leave the brownies to cool before cutting up into squares. Simply have them on their own or with an afternoon cup of coffee. Enjoy!