Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.
Step 2: Pour the dry German chocolate cake mix evenly into the bottom of the prepared baking dish. Do not prepare the cake mix according to package directions; use it dry.
Step 3: Drizzle the entire can of sweetened condensed milk evenly over the dry cake mix, trying to cover as much of the surface as possible.
Step 4: Carefully spoon dollops of the coconut pecan frosting over the sweetened condensed milk layer. You don't need to spread it perfectly; it will melt and spread as it bakes.
Step 5: Arrange the thin pats of cold butter evenly over the coconut pecan frosting layer.
Step 6: Bake for 45-55 minutes, or until the edges are bubbly, the topping is golden brown, and a wooden skewer inserted into the cake part comes out mostly clean (a little goo is desirable for this type of cake).
Step 7: Remove from the oven and let cool on a wire rack for at least 15-20 minutes before serving. This allows the gooey layers to set slightly. Serve warm with ice cream or whipped cream, if desired.