Preheat the oven to gas mark 4/350oF/180oC/160oC fan then line an 8 x 8-inch (20 x 20cm) tin with parchment paper. Ensure you leave some overhang to make it easier to remove the brownies.
To a bowl, add and combine the cocoa powder, plain flour and salt then set aside.
To a heat resistant bowl, add the butter and chocolate. Place the bowl over a pot of simmering water on a low heat ensuring the bottom of the bowl doesn’t touch the water.
Using a spatula, mix the butter and chocolate while they melt. Once melted, immediately remove from the heat. The bowl will be warm so do not remove with your bare hands.
To the bowl with the melted butter and chocolate, mix in both sugars, the eggs, vanilla extract and vegetable oil. Add and combine the dry ingredients set aside earlier and then add the chocolate chips.
Pour the batter into the baking tin and bake for 30 minutes. Remove from the oven and place the tin on a cooling rack to completely cool. Once cooled, cut into 16 pieces and enjoy.