Cube your butter and break your dark chocolate into chunks adding to a bowl. Fill a small saucepan with hot water, about a quarter of the way full, on a low heat.
Put your butter and chocolate bowl on top of the pan, making sure it doesn't touch the water and allow to melt, stirring until combined. Once fully melted, set to the side to cool.
Pre-heat your oven to 180C/160C fan/gas 4.
Line a shallow baking tray with butter and baking paper and set aside.
In a new bowl break your eggs and add your Tate & Lyle Fairtrade Pure Cane Golden Caster Sugar and Tate & Lyle Fairtrade Pure Cane Dark Brown soft sugar, with a hand mixer, cream the eggs and sugar before combining with your butter and chocolate mix.
Sieve your flour and cocoa powder together with a pinch of salt and fold into your sugar, eggs, chocolate and butter mix.
Folding in gently and adding in your broken up chunks of milk chocolate.
Pop in the oven for 35 - 45 minutes and allow to fully cool before slicing.