Preheat oven to 325°F and grease a 9×13 pan.
Mix cake mix, melted butter, brown sugar, cinnamon, sour cream, and eggs until smooth.
Pour half the batter into the pan.
Mix brown sugar, cinnamon, and melted butter to make the filling.
Spoon filling over batter.
Add remaining batter on top; swirl gently.
Bake 30–40 minutes, until set but still soft.
Whisk powdered sugar, milk, and vanilla; pour over warm brownies.
Cool slightly before slicing.