Preheat oven to 350. Grease a 9×13 pan.

Mix cake mix, oil, water, eggs, and sour cream until smooth.

In a separate bowl, mix together brown sugar, cinnamon, and pecans (if using).

Pour half of cake mix into pan. Then top with half of brown sugar mixture. Repeat with remaining cake mix and brown sugar mixture.

Using a knife, swirl cake mix and brown sugar mixture to your heart’s content.

Bake at 350 for about 35 minutes.

Make the glaze: whisk powdered sugar, milk, and vanilla together. Pour over warm cake.

Enjoy!