Put the butter, brown sugar, corn syrup and condensed milk in a medium saucepan. Bring to a boil over medium heat, while you consistently stir.
Once it begins to boil, very carefully add the whipping cream and stir it in.
Continue to stir the mixture over medium heat, until it reaches 238 degrees (soft ball candy stage). I absolutely love my spatula thermometer because it allows me to stir AND see the temperature without burning my hand.
Once it reaches 238 degrees, remove it from the heat and stir in the vanilla, salt, and the hot cocoa powder.
Carefully stir in the marshmallows just before pouring.
Just pour and spread the hot caramel on a baking sheet that has been sprayed lightly with oil.
Once cool to touch, cover with plastic wrap or foil, and place in the fridge for an hour (or freezer for half the time)
Cut the pieces into about 2-bite size candies, and wrap them in wax paper. You could also roll them into little balls (as I did).
Be prepared to make multiple batches, because they WILL be requested if you share them!