Preheat your oven to 350 degrees F (175 degrees C). Lightly grease and flour a 9×13 inch baking pan.
Prepare the cake batter according to the package directions, substituting the water with 1 cup of water, oil, eggs, and vanilla extract. Stir in the 1/2 cup toasted pecans.
Pour the batter into the prepared pan and bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
While the cake bakes, prepare the soaking liquid. In a medium bowl, whisk together the butterscotch pudding mix and 1 cup of boiling water until smooth. Whisk in the cold milk. Set aside.
While the cake is still warm, use the handle of a wooden spoon or a fork to poke holes all over the top, about 1 inch apart.
Slowly pour the butterscotch mixture evenly over the top of the warm cake, allowing it to soak into the holes. Let the cake cool completely on a wire rack.
Prepare the praline sauce. In a small saucepan, combine the sweetened condensed milk, brown sugar, butter, and 1/2 cup heavy cream.
Cook over medium heat, stirring constantly, until the mixture comes to a boil and thickens slightly, about 5 to 7 minutes. Remove from heat and stir in 1 teaspoon vanilla extract.
Pour the warm praline sauce evenly over the cooled cake. Refrigerate for at least 2 hours, or until the sauce sets.
Before serving, spread the thawed whipped topping over the praline layer. Garnish with the remaining 1/4 cup chopped pecans.
Notes
To toast pecans, spread them on a dry skillet over medium heat for 5 to 7 minutes, stirring often until fragrant.
For a richer flavor, you can substitute the instant pudding with a homemade custard base.
If you prefer a thicker topping, use a stabilized whipped cream or cream cheese frosting instead of frozen whipped topping.