Preheat oven to 350 degrees F. Grease a 9-inch square baking pan; set aside.
Make the Filling: Remove ⅔ cup of the cake mix and place in a medium bowl. Add the melted butter, brown sugar, cinnamon, and chopped pecans, mixing with a spoon or spatula to thoroughly combine.
In a large bowl, combine the remaining cake mix, eggs, and sour cream until completely combined.
Pour half of the wet batter into the prepared pan, then sprinkle evenly with half of the filling.
Pour the remaining batter over top and smooth it into an even layer. Sprinkle the remaining filling mixture over the top of the batter.
Bake for 45 minutes to 1 hour, or until a toothpick or thin skewer inserted into the center comes out clean. Allow to cool for at least 15 minutes before serving to serve warm; it can also be served at room temperature.
Notes
Yellow Cake Mix: Use your favorite brand here. Using a mix is so quick and easy! I also have a recipe for a DIY yellow cake mix if you want to try making your own!
Pecans: You could swap for walnuts if you prefer, or omit entirely if you need to avoid nuts.
Sour Cream: Use full-fat for a wonderfully moist cake. You can also substitute full-fat Greek yogurt.
Serving: Allow to cool for at least 15 minutes and serve warm or at room temperature.
Garnish: Serve plain or with a sprinkle of powdered sugar or a drizzle of simple icing (mix powdered sugar with water or milk to create a drizzling consistency).
Storage: Cover with plastic wrap and store at room temperature for up to 2 days.