You won't need an oven for this one.
1 cup whole milk14 oz. dark chocolate , broken into small piecesHeat the whole milk, sugar, baking soda, and pinch of salt in a heavy-based saucepan over high heat. Just before the mixture comes to the boil, reduce the heat to very low, ensuring that the mixture maintains a very gentle simmer . Cook, stirring often, for about 1 hour, in which time the mixture will darken and thicken. From this point, stir the mixture frequently to avoid it catching and burning.
Dip one-third of the biscuits in the cocoa milk for just a few seconds, then transfer to the base of the prepared tin.Spread one-third of the dulce de leche mixture over the biscuits, then repeat with two more layers of dipped biscuits and dulce de leche. Set aside in the fridge.To make the ganache, place the cream in a saucepan over medium heat. When the cream is warm, but not boiling, remove from the heat and add the chocolate but do not mix or beat.