Heat the oven to 350°F and grease a 9-by-13-inch baking dish. In a large bowl, combine the yellow cake mix and instant vanilla pudding mix.

Add the eggs, vegetable oil, and sour cream to the dry mixture. Beat or stir just until the batter is smooth and fully combined, taking care not to overmix.

In a separate bowl, stir together the packed brown sugar, ground cinnamon, and chopped pecans or walnuts. This mixture will create the sweet, warmly spiced ribbon and crunchy topping.

Pour half of the thick batter into the prepared baking dish and gently spread it into an even layer. Sprinkle about half of the cinnamon-nut mixture over the batter.

Add the remaining batter and spread it carefully over the cinnamon layer. Finish by sprinkling the remaining cinnamon-nut mixture evenly across the surface.

Bake for 45–50 minutes, or until the center is set and a toothpick inserted into the middle comes out with only a few moist crumbs. Cool the cake for at least 15–20 minutes before slicing.