Prepare cake according to instructions on the box. Allow to cool.
In a medium bowl, mix together butter, sugar, sweetened condensed milk, cinnamon and vanilla.
Poke holes throughout the cake using the handle of a wooden spoon.
Pour filling over the holes in the cake.
In a medium bowl, use an electric mixer to beat heavy cream until stiff peaks form. Set aside.
In a large bowl, beat cream cheese and butter together.
Add confectioners' sugar, vanilla extract and salt.
Fold in whipped cream from medium bowl.
Frost the cake with the frosting.
Sprinkle some cinnamon sugar on top.
Refrigerate 30 minutes before serving.