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, Milo powder 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 30 minutes or until cooked through. Leave in the tin to cool completely.
To make the mousse, place the dark chocolate into the Thermomix bowl and mix for 5 seconds, Speed 7. Set aside.
Insert the butterfly into the Thermomix bowl. Add the thickened cream, Milo, cocoa powder and caster sugar and mix for 20 seconds, Speed 4.
Add grated chocolate and beat for 10 seconds, Speed 4. If the mousse has not thickened, continue to mix in 5 second bursts until thickened (do not over-mix).
Spread the chocolate mousse over the cooled brownie. Place into the fridge for 5 hours to set and then sprinkle with extra Milo before serving.
Store in an airtight container in the fridge for up to 5 days.