Preheat your oven to 325 degrees and spray a 9x13-inch baking dish with cooking spray. This step is crucial for preventing the cake from sticking, ensuring you have easy slices later.
In a large bowl, stir together the cake mix, eggs, vegetable oil, and sour cream until smooth. You’ll want a batter that’s well combined and creamy. The texture should be thick yet pourable.
Pour half of the cake batter into the prepared pan and smooth it out evenly. Take your time to ensure it’s level; this will help the cake bake uniformly.
In a separate bowl, stir together the brown sugar and cinnamon. This mixture is what creates the delicious swirl in the cake. Sprinkle it evenly over the cake batter, ensuring every bite will have that sweet flavor.
Cover the cinnamon mixture with the remaining cake batter, spreading it evenly. This layers the sweetness beautifully.
Using a knife, run it through the batter in a swirling pattern. This creates a delightful marble effect, ensuring the cinnamon is distributed throughout the cake.
Bake for 30 to 35 minutes. You know it’s done when the edges are golden brown and a toothpick inserted in the center comes out clean. The smell will be heavenly!
In a medium bowl, whisk together powdered sugar, milk, and vanilla to create the glaze. It should be pourable yet thick enough to hold its shape.
Once the cake is done, pour the glaze over it while it’s still warm. This allows the glaze to seep into the cake, adding an extra layer of sweetness.
Let the cake cool for 20 minutes before cutting. This resting time lets the flavors meld beautifully and makes slicing easier.