Preheat the oven to 350 degrees F. Lightly coat a baking sheet (13x18x1) with non stick baking spray and set aside.
Combine the cake mix, vanilla pudding mix, eggs, milk, oil and sour cream in a large bowl and beat on low speed for 30 seconds, then medium speed for 1 1/2 minutes, scraping the sides of the bowl as needed. Pour the cake mix into the prepared pan.
In a small dish combine the melted butter, brown sugar and cinnamon until smooth. Drizzle over the cake, then use a butter knife to swirl in the brown sugar mixture.
Bake for 20 to 25 minutes or until toothpick inserted in center comes out clean.
Once finished baking, mix together the ingredients for the icing. Add more milk to thin the icing, more powdered sugar to make it thicker. Drizzle over the warm cake, slice and enjoy!