Step 1: Preheat Your Oven - Preheat your oven to 350°F (175°C).
Step 2: Prepare the Baking Pan - Grease your 9x13-inch baking pan with cooking spray or butter.
Step 3: Mix the Cake Batter - In a large mixing bowl, combine the yellow cake mix, eggs, vegetable oil, and sour cream. Blend until smooth.
Step 4: Create the Cinnamon Swirl - In a separate bowl, mix the brown sugar and cinnamon together.
Step 5: Layer the Cake Batter - Pour half of the cake batter into the pan, sprinkle half of the cinnamon mixture, pour the remaining batter, and finish with the rest of the cinnamon mixture. Swirl together.
Step 6: Bake the Cake - Bake for 35 to 40 minutes. Check with a toothpick.
Step 7: Prepare the Glaze - Whisk together the powdered sugar, milk, and vanilla extract until smooth.
Step 8: Glaze the Cake - Drizzle the glaze over the cake once cooled slightly.