Preheat oven to 325 degrees. Combine cake mix, eggs, sour cream and oil in a large bowl and whisk together until well blended; set aside.
Combine brown sugar and cinnamon in a separate bowl. Spread half of the cake batter into a greased 9x13-inch cake pan.
Sprinkle with half of the brown sugar and cinnamon, covering the entire cake. Spread the rest of the cake batter over layer and top with remaining sugar and cinnamon.
Use a knife and swirl the batter. Don't swirl too much though, because you want to have pockets of cinnamon and sugar.
Bake for around 45 minutes or until a toothpick inserted in the center comes out clean.
Meanwhile, make the glaze. Combine powdered sugar, one Tablespoon of milk and vanilla and stir until smooth. If it's too thick, add a little more milk. Pour over the hot cake right when it comes out of the oven. Let set.
Serve warm or at room temperature. You can add 1/3 cup of chopped pecans to the brown sugar and cinnamon mixture if you would like.