1. Preheat your oven to 160 °c. Grease and line a cake tin {20-22cm in diameter}.
2. Place butter and brown sugar into the Thermomix bowl and mix on Speed 6/30 seconds, or until creamy. Scrape down the sides of the bowl using the spatula.
3. Add the eggs, bicarb soda, milk, flour, baking powder and cocoa to the Thermomix bowl. Mix on Speed 6/1 minute.
4. Pour the mixture into the cake tin, and place into the oven. Bake for 1 hour and 10 minutes, or until cooked.