Heat the oven to 350°F (175°C). Grease and lightly flour a 9 x 13-inch baking dish, or coat it thoroughly with baking spray.

In a large bowl, mix the yellow cake mix, vegetable oil, eggs, and sour cream until smooth and just combined. Do not beat the batter longer than necessary.

Spread approximately half of the thick batter in the prepared baking dish. Smooth it gently into an even layer.

Stir the packed brown sugar and cinnamon together, then sprinkle the mixture evenly over the batter. Add chopped pecans here if desired.

Spoon the remaining batter over the cinnamon layer and spread it carefully with a greased spatula or the back of a spoon.

Lightly swirl a knife through the top layers without fully mixing them. Bake for 38 to 45 minutes, until the center is set and a toothpick inserted into the cake portion comes out clean.

Whisk the confectioners' sugar, milk, and vanilla until smooth. Pour the glaze over the warm cake, then let it cool for at least 15 minutes before slicing.