Preheat the oven to 180C. Line a 20cm tin with non-stick baking paper.
Put the butter and dark chocolate in a microwave-proof bowl, cover with cling film and heat on high for 2 minutes until melted. Alternatively you can heat it over a saucepan of simmering water. Leave to cool.
Put the eggs and sugar in a mixing bowl and whisk for at least 5 minutes until the mixture is pale and about twice its original size. Pour the melted and cooled chocolate and butter mixture into the mixing bowl and fold in until combined. Don't overmix. Stop as soon as you think it is all mixed in.
Add the flour and cocoa powder by holding a sieve above the mixing bowl and sift the flour and cocoa into the chocolate mixture. Fold this in as well. Again, stop as soon as it is combined and there are no dry floury bits.
Fold in the chocolate chips.
Then put the mixture into the tin.
Bake in the oven for 25 minutes. To check if it's ready, give the tin a gentle shake. If it's still quite wobbly, return to the oven for another 5 minutes and check again. There should be a crust on top and it will be coming away from the sides of the tin a little. Let it cool in the tin before cutting into squares.
Nutritional information is approximate and a guideline only.