Preheat oven to 170C fan-forced (190C conventional). Grease and line an 8-cup capacity baking dish.

Add the butter and one 180g block of Caramilk, broken into pieces, to a saucepan and place over low heat. Stirring constantly, cook until the butter and chocolate have melted and are thoroughly incorporated. Remove from heat and allow to cool slightly before adding the brown sugar, vanilla and eggs. Whisk again. Dump in the rest of the pudding ingredients and whisk until smooth. 

Break the additional block of Caramilk into chunks and throw them haphazardly across the base of your cooking dish (this chocolate will melt and become part of the sauce while it cooks in the oven). Spoon the batter on top of the chocolate chunks. 

Add the sauce ingredients to a large heatproof pouring jug. Whisk until the butter has been melted by the boiling water. Pour the sauce over the batter, keeping a tight grip on the edges of the dish, so none of the sauce sloshes over. Don't be nervous – it's going to look hideous and weird, and the pudding mixture will float and rivers will run through the batter and you'll think it's ruined, but I promise it's not. 

 Pop in the oven for 30 minutes or until the top looks lightly golden and set. 

Allow to cool slightly so the sauce thickens in the base of the cake. Dust with icing sugar and serve warm, with or without vanilla ice-cream.