Preheat oven to the temperature specified on the cake mix box.
Prepare the chocolate cake according to the package instructions. Pour batter into a greased 9×13 inch baking dish.
Bake as directed on the cake mix box. Remove from oven and while still hot, use the end of a wooden spoon to poke holes all over the top of the cake.
In a microwave-safe bowl, combine sweetened condensed milk and chocolate chips. Microwave until chips are melted, stirring every 30 seconds.
Pour the chocolate mixture over the hot cake, making sure to fill the holes. Let the cake cool completely.
For the topping, whip the heavy cream, sugar, and vanilla extract until stiff peaks form. Spread over the cooled cake.
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