Preheat the oven to gas mark 4/350oF/180oC/160oC fan and line a 13 x 9-inch baking tin with parchment paper ensuring you leave some overhang. Start by sifting the cocoa powder, plain flour and salt into a bowl then whisk and set aside.
In a microwaveable bowl, melt the butter and chocolate in 40 second increments stirring each time. Once melted, mix well and set aside.
In the third bowl, pour in the brown sugar, granulated sugar and whisk them together. Whisk the eggs into the sugar one at a time, stirring between each egg. Add the vanilla extract and vegetable oil then mix until the mixture is smooth and creamy.
Pour the melted chocolate into the bowl with the wet ingredients and mix well then fold in the dry ingredients that you set aside earlier. Mix in the chocolate chips and combine well.
Pour the brownie batter into the baking tin and bake for 35 - 40 minutes. Once baked, allow to cool before you cut them into 20 squares.