Begin by preheating your oven to 160°C / 320°F. Grease a 20cm square cake tin with oil spray and line the sides and bottom with baking paper. Set aside.
Add the butter and chocolate into a heatproof bowl and microwave for 20 seconds at a time, stirring each time until completely melted and smooth. Alternatively, you can melt these two ingredients together by placing the bowl over a saucepan of simmering water. (don’t let the bottom of the bowl touch the water.
Add the whisked eggs and mix them into the chocolate/butter mixture. Then add the cocoa powder, salt, sugar, vanilla extract and flour and mix until well combined. Finish by adding the chocolate chips, fold through using a spatula. Pour the batter into the square baking tin and spread out evenly using a spatula. Bake for 25 minutes, take out of the oven and allow to cool completely before slicing up and serving.
Brownies can be stored in an airtight container for up to three days and are even better the next day!