Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking pan.
Prepare the cinnamon filling: In a small bowl, mix brown sugar and cinnamon. Set aside.
Make the cake batter: In a large bowl, mix the cake mix, sour cream, oil, eggs, milk, and vanilla extract until smooth.
Layer the cake: Pour half of the cake batter into the pan and spread it evenly. Sprinkle the cinnamon sugar mixture on top. Pour the remaining batter over and spread gently.
Bake for about 40 minutes or until a toothpick inserted in the center comes out clean. Your kitchen will smell amazing!
Prepare the glaze: In a small bowl, whisk powdered sugar, milk, and vanilla until smooth.
Add the glaze: Once the cake is slightly cooled, drizzle the glaze over the top. Let it set before slicing.