In a large bowl, beat cake mix, chocolate milk, canola oil, and eggs until well combined (about 2 minutes). Pour into greased pan.
Bake for 34-36 minutes or until a toothpick inserted into the center comes out clean. Let cool for 10 minutes.
Poke holes all over the top of the cake with a fork.
In a medium bowl, whisk sweetened condensed milk, half and half, evaporated milk, and 3 tablespoons cocoa powder until cocoa powder is fully mixed into the milks. Pour this evenly over the cake. Refrigerate for at least 4 hours or overnight.
Topping
Make the topping by first sifting the powdered sugar and cocoa powder together in a medium bowl.
Make whipped cream by beating heavy cream in a stand mixer on medium speed for 2 minutes. Add half the sugar/cocoa mix and beat until incorporated. Add the rest of the sugar/cocoa mix and beat on medium-high heat until soft peaks form.
Spread chocolate whipped cream over the cake right before serving as well as chocolate curls.