Preheat your oven to 160 degrees and line the base and sides of a 20cm baking tin with baking paper.
In a small saucepan, melt the butter, cocoa and sugar over a low heat - stirring continuously.
Transfer melted ingredients to a large mixing bowl and add the vanilla essence and egg whisking well.
Sift the flour over the mixture and whisk to combine.
Spoon half the mixture into the cake tin and carefully place the Twix Bars on top.
Add the remaining brownie mixture to cover the Twix Bars and pop into the oven and bake for 30 minutes.
Allow Brownie to cool in the tin before removing, cut Brownie into small (or large!) pieces and enjoy!