Preheat your oven to 180°C (gas mark 4, 350°F) and line a baking tin with parchment paper.
Add butter and dark chocolate into a mixing bowl and heat in the microwave until melted.
Pour caster sugar, eggs and vanilla extract into a large mixing bowl. Using an electric whisk, whisk the mixture until it becomes frothy, pale and has doubled in volume.
Pour the chocolate and butter mixture into the whisked eggs mixture and gently fold using a spatula until evenly combined.
Sift plain flour and cocoa powder and fold with your spatula until evenly combined.
Take the Crunchie chocolate from the fridge or freezer and cut into 1″ (2.5cm to 3cm) large chunks. Place into the mixing bowl and mix until evenly distributed.
Pour the brownie mixture into a lined baking tin and spread evenly with a spatula. Then bake in the oven for 25 minutes.
Leave to cool down for at least 20 minutes for gooey brownies, for fudgy brownies leave to cool for at least 45 minutes in the fridge before cutting into slices.
Video
Notes
You can decorate the brownies by sprinkling extra chopped Crunchie bars on top.