For the dulce de leche:
4 cups whole milk
10½ oz. (300 g) granulated sugar
¼ tsp. baking soda
Pinch of salt
For the cake
Butter, for greasing
1 lb 2 oz. (500 g) cream cheese
1 lb 2 oz. (500 g) Dulce de leche
1 Tbsp. unsweetened (Dutch) cocoa powder
1 cup whole milk
14 oz. (400 g) Chocolinas or plain uncoated chocolate cookies
For the ganache
7 fl. oz. (200 ml) heavy cream
14 oz. (400 g) dark chocolate (at least 70% cocoa solids), broken into small pieces
To make the dulce de leche:
Heat 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 (you may need to adjust the heat accordingly). 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. Continue stirring for 20–30 minutes, until the mixture is thick and toffee-colored.
To test if the dulce de leche is ready, place a spoonful on a cold saucer or plate. Allow it to cool and thicken, then run your finger through the centre of the dulce de leche. If the mixture doesn’t pool back, it is ready. Transfer to a bowl and allow to cool completely.
Transfer the dulce de leche to a very clean and dry glass jar. It will keep in the fridge for 2–3 weeks.
To make the cake:
Lightly grease a 8¾-in springform cake tin and line the base and side with baking paper.
Place the cream cheese and dulce de leche in a bowl and use electric beaters to beat until well combined. Set aside.
Whisk the cocoa powder and milk in a bowl, until the cocoa is dissolved. 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. Set aside for 10 minutes, then gently stir until smooth and glossy. Pour the ganache over the chocotorta and return to the fridge for at least 6 hours or overnight to set.
Remove the chocotorta from the tin and gently remove the baking paper. To serve, warm a knife in hot water, then dry and cut the cake into thick slices.
Notes:
Dulce de leche can also be made in the oven. Preheat the oven to 350°F. Pour two 14-oz tins of sweetened condensed milk into a 1 liter (4 cup) baking dish and cover with foil. Sit the baking dish in a larger dish and pour enough boiling water to come half way up the sides of the dish with the condensed milk. Cook in the oven, topping up with boiling water to maintain the level, for 2 hours, or until the condensed milk is toffee-colored (the top will be darker). Carefully remove from the oven and stir to combine while still warm. Set aside to cool completely.