Preheat oven to 150 degrees celsius (fan-forced). Grease and line the base and sides of a 20cm round cake tin with baking paper. Set aside.
Place the chopped butter, brown sugar, milk and Caramilk chocolate (broken into pieces) into the Thermomix bowl and melt for 4 minutes, 60 degrees, Speed 2 (or until melted and smooth).
Add the plain flour, self-raising flour and add the vanilla extract, eggs and salt and mix for 5 seconds, Speed 5. Scrape down the sides of the bowl and repeat for a further 5 seconds (or until smooth and combined). Pour the mixture into the prepared cake tin.
Bake for 60 minutes or until a skewer inserted into the middle comes out with just a few crumbs on it. Allow the cake to cool before transferring to a wire rack or plate.
To make the Caramilk ganache, place the Caramilk chocolate (broken into pieces) and the thickened cream into the Thermomix bowl. Melt for 3 minutes on 60 degrees, Speed 2 (or until the chocolate has melted). Stir until smooth. Place ganache into the fridge for 1-2 hours (stirring occasionally) until thickened. Spread the Caramilk ganache over the top of the cake and serve.