First, heat your oven to 350°F (175°C). While the oven is warming up, add the cake mix, eggs, vegetable oil, and water to a large bowl. Stir everything together until the batter is smooth and well blended, with no dry spots.
Grease a 9×13-inch baking pan and pour the batter evenly into the pan. Place it in the oven and bake for 30 to 35 minutes. The cake is done when a toothpick inserted into the center comes out clean.
Take the cake out of the oven and let it rest for about 10 minutes. While it is still warm, use the handle of a wooden spoon or a skewer to poke holes all over the cake, spacing them evenly.
In a separate bowl, mix the melted butter and sweetened condensed milk until combined. Stir in the toasted, chopped pecans to make the butter pecan filling.
Slowly pour this mixture over the warm cake, allowing it to sink into the holes. Let the cake cool completely so the filling can soak in and set properly.
To make the frosting, whisk the cold milk and instant butterscotch pudding mix in a medium bowl for about 2 minutes, until it thickens. Gently fold in the whipped topping and chopped pecans until the frosting is light and smooth.
Once the cake is completely cool, spread the frosting evenly over the top. Refrigerate the cake for at least 1 to 2 hours before serving so the flavors come together and the texture is just right.