Use half of the Betty Crocker milk chocolate cake mix to make a half base. Prepare as instructed on box with with egg, oil and water. Pour batter into a prepared 23x33-cm cake pan and bake in a 180°C preheated oven until it is fully cooked for about 35 minutes.
Remove cake from oven and cool at room temperature. Then place it in the freezer for about 1 hour or more until it hardens.
Pour boiling milk all at once into the blended custard, stirring well.
Let your custard cool down for a few minutes at room temperature, then pour it on top of your frozen chocolate cake sponge and return to freezer. While waiting for the custard layer to harden in the freezer, start preparing your caramel sauce.
To prepare caramel sauce, mix together the brown sugar, cream, butter and salt in saucepan over a medium-low heat. Cook while whisking gently for 5 to 7 minutes, until it gets thicker. Add vanilla and cook another minute to thicken further. Turn off the heat when finished.