Preheat oven to 350 degrees F. Grease a 9-inch square baking pan; set aside.: The kitchen fills quickly with warmth, and you may notice the faint scent of metal warming from the pan, signaling readiness. Greasing the pan well prevents sticking and encourages an even edge texture, which matters when you slice into those neat squares. A common mistake is not greasing corners thoroughly, leading to bits that cling to the pan, so run a thin layer of butter into each corner and along the sides. If your oven runs hot, watch early for browning around 30 minutes so the cake does not over brown.
Make the Filling: Remove 2/3 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.: As you stir you will smell the warm notes of brown sugar and cinnamon mingling, and the sticky filling will glisten slightly from the melted butter . Mixing by hand gives you control so the pecans remain defined rather than pulverized. This filling creates pockets of caramel like sweetness inside the cake, which is why we remove that 2 3rd cup of dry mix rather than adding other flour. One pitfall is adding too much butter , which makes the filling runny and can cause it to sink; aim for a moist crumbly texture that holds shape when scooped.
In a large bowl, combine the remaining cake mix, eggs, and sour cream until completely combined.: When you mix the batter it should feel smooth and thick, with the sour cream adding silkiness that cuts through the sweetness. Using room temperature eggs helps them integrate cleanly and prevents cold spots that could break the emulsion. Take a moment to scrape the bowl sides so no pockets of dry cake mix remain, which would create dense streaks in the finished cake. Avoid over mixing, which can develop gluten and lead to a tougher crumb.
Pour half of the wet batter into the prepared pan, then sprinkle evenly with half of the filling.: As you dollop the batter you will see a pale, creamy layer settle into the pan; when you evenly scatter the filling it will create ribbons of darker, sticky goodness. Press gently with the back of a spoon to level without compacting the batter. The contrast between batter and filling is visually satisfying and guarantees pockets of texture in every bite. If you pile filling in one spot it may sink excessively during baking, so spread it deliberately.
Pour the remaining batter over top and smooth it into an even layer. Sprinkle the remaining filling mixture over the top of the batter.: Smoothing the top gives the cake an even rise and helps the topping bake into a crumbly crust rather than a collapsed mound. When the filling is sprinkled on top it will crisp and caramelize, releasing a toasty scent; watch for any overly large clumps and break them up so the distribution is uniform. A common error here is pressing the topping into the batter, which buries the crunchy bits; keep the filling as a loose layer on top.
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.: The oven will yield a golden top and a fragrant, cinnamon filled aroma. When you insert a toothpick you want it largely clean with maybe a few moist crumbs, indicating a tender interior, rather than wet batter which means it needs more time. Cooling slightly lets the crumb set so slices hold together, and the first warm slice will steam gently, releasing more aroma. A typical misstep is underbaking to chase softness; if the center jiggles gently continue to bake in 5 minute increments until set.