Place the butter into the Thermomix bowl. Melt for 3 minutes on 80 degrees, Speed 2. Add the caster sugar and cocoa powder and mix for 10 seconds on Speed 4. 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 for 10 seconds on Reverse, Speed 4. Scrape down the sides and repeat. Add the plain flour and mix for 5 seconds on Speed 4. Scrape the sides of the bowl and repeat.
Pour the brownie mixture into the prepared tin and bake for 25-30 minutes at 160 degrees C or until cooked through. Leave in the tin to cool completely.