Prepare the Oven and Pan: Preheat the oven to 350°F. Grease a 9×13-inch baking dish. Preheating ensures even baking. Greasing prevents sticking.

Mix the Cake Ingredients: In a large mixing bowl, combine the cake mix, frosting, eggs, oil, and milk. Blend until smooth with an electric mixer. Blending ensures a smooth batter without lumps.

Fold in Pecans: Fold in 1/2 cup of finely chopped pecans. Folding gently prevents crushing the pecans.

Transfer the Mixture: Transfer the mixture to the prepared baking dish. Transferring evenly ensures uniform baking.

Bake: Bake for 30-40 minutes, or until a toothpick inserted in the center comes out clean. Baking until a clean toothpick ensures the cake is fully cooked.

Prepare the Cake for Topping: After baking, while still warm, poke holes across the cake with the handle end of a wooden spoon. Poking holes allows the sauce to seep into the cake, enhancing flavor.

Make the Sauce: In a saucepan over medium heat, melt the butter. Add the sweetened condensed milk and stir until well combined. Melting and combining creates a smooth and homogenous sauce.

Simmer and Fold in Pecans: Bring to a simmer, then fold in 1/2 cup of finely chopped pecans. Simmering thickens the sauce. Folding in pecans adds texture.

Finish the Cake: Pour the sauce over the warm cake, ensuring it seeps into the holes. Pouring evenly ensures the sauce is distributed throughout the cake.

Cool and Serve: Allow the cake to cool to room temperature before serving. Cooling allows the sauce to set and the flavors to meld.