Preheat oven to 180°C on a fan forced setting. Place a large tray of water in the oven on the lowest rung. Butter a large rectangular baking dish, preferably ceramic or several ramekins as needed. Set aside.
To make cake, place dark brown sugar and butter into TM bowl and melt together 3 min/100°C/speed soft.
Add remaining cake ingredients and blend 20 sec/speed 5-6. Scrape down sides of bowl and blend a further 10 sec if needed.
Scrape into prepared baking dish and set aside.
Without cleaning bowl, place all sauce ingredients into TM bowl and blend gently (to avoid hot splashback) 3 sec/speed 3-4.
Pour sauce over the back of a serving spoon onto the cake and bake in the water bath for 40-45 minutes until golden and bubbling.