Prepare chocolate cake batter according to the package directions by adding egg, oil, and water.
Pour batter into a prepared 23x33-cm cake pan and bake as directed. Remove from oven when finished.
While the cake is still hot, use a fork to pierce holes in the top. Since a warm cake tears easily, remember to work carefully.
In a medium sized bowl, mix together the sweetened condensed milk and the caramel sauce. Gradually pour this mixture over the warm cake.
To prepare caramel sauce, mix together 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 heat when finished.
Allow cake to cool to room temperature, and then place in the refrigerator for 30 minutes or more to chill.
When well chilled, remove cake and spread generously with whipped topping. Then, sprinkle with the crushed candy bar.