Step 1

Preheat the oven to 350 degrees Fahrenheit. Grease a 9x13-inch cake pan with cooking spray or butter.

Step 2

In a large mixing bowl, beat together the yellow cake mix, eggs, sour cream, vegetable oil, and water until smooth and well-combined. In a separate bowl, whisk together the brown sugar and cinnamon.

Step 3

Pour half of the batter into the greased baking pan. Sprinkle the top with the prepared sugar-cinnamon mixture. Pour the remaining batter over. Gently drag a table knife through the batter to create ripples.

Step 4

Bake the cake for about 40 minutes, or until lightly golden on top and a toothpick inserted into the center comes out clean.

Step 5

Meanwhile, whisk together powdered sugar, milk, and vanilla until smooth. Pour it over the cake while still hot.

Step 6

Let the cake cool on a wire rack for at least 15 minutes. Chill until ready to serve, or serve warm with cold milk or a scoop of ice cream.