Preheat oven to 160 degrees fan. Grease and line a deep 9 inch cake tin with baking paper.
Melt together the NI unsalted butter, dark and milk chocolate in a microwave safe bowl for 1-2 minutes until melted, stirring every 30 seconds. Leave to the side to cool.
Using a hand whisk or stand mixer, whisk together the eggs, vanilla and caster sugar for about 5-6 minutes until pale and frothy.
When it’s ready the mix should leave a trail for a couple of seconds.
Pour the cooled chocolate mixture into the whisked eggs and fold together carefully with a spatula. Be patient – you don’t want to knock out the air you added!
Once fully combined, sift in the cocoa powder and plain flour and fold together again gently.
Once everything is combined, fold through the chocolate chips and pour into the tin – bake in the oven for 35-45 minutes or until a skewer comes out mostly clean. (They shouldn’t wobble when you shake the tin)
Once baked, leave to cool in the tin fully. Refrigerate overnight to make them super fudgey.
Use a hot knife to cut the brownies for a nice clean cut. I cut into 10 triangles.
They will last in an airtight container for about 1 week but I doubt they’ll last that long!