Preheat oven to 160 degrees celsius (fan-forced) and line a 20cm square tin with baking paper. Set aside.
Place the butter into the Thermomix bowl. Melt on 80 degrees, Speed 2, 3 minutes.
Add the caster sugar and cocoa powder and mix on Speed 4, 10 seconds. Scrape down the sides of the bowl and repeat for a further 10 seconds. Allow to cool slightly.
Add the vanilla extract and the eggs and mix on Reverse, Speed 4, 10 seconds. Scrape down the sides and repeat.
Add the plain flour and mix on Speed 4, 5 seconds. Scrape the sides of the bowl and repeat.
Pour the brownie mixture into the prepared tin and bake for 25-30 minutes or until cooked through. Leave in the tin to cool completely.