Make the cake per instructions of back of box, using a 9x13 baking dish, sprayed lightly. Allow cake to cool completely.
Filling: In a medium bowl, whisk together sweetened condensed milk, brown sugar, butter, cinnamon, and salt.
Using the bottom of a wooden spoon, poke lots of holes all over the cake. Pour the filling over the holes and cake. Using a spatula, spread evenly making sure to push filling into holes.
Icing: In a medium bowl, using a hand mixer beat heavy cream until stiff peaks form.
In another bowl, beat cream cheese and butter together until no lumps. Add powdered sugar and continue to beat until smooth. Add vanilla and salt and mix.
Fold in whipped cream until combined. Spread icing all over cake.
Refrigerate 20 minutes before serving.
