Combine milk, heavy cream, sugar, Kosher salt, and cinnamon stick in a medium saucepan, then stir. Bring to a boil.
Pour into the prepared pan, then use a spatula to spread evenly.
Sprinkle chopped chocolate (or chocolate chips) and chopped pecans evenly on top.
Pour the dry cake mix on top, smoothing into an even layer with a spoon or spatula, pour on the melted butter, trying to cover all of the cake mix with butter.
Place the cake in a preheated oven, then bake until golden brown and puffed up. Cool slightly.
Serve warm with cinnamon whipped cream, ice cream, or caramel sauce.