Prepare the cake batter according to the package instructions and place it in a 24 cm diameter hole-in-the-middle pan.
In a medium saucepan, cook the condensed milk with the chocolate powder over medium heat. Stir until it thickens slightly and begins to unglue from the pan. Remove from heat, add the cream and mix until smooth.
Spread the syrup over the cake, leaving enough frosting in the center hole, to have the “volcano” effect when it is cut.