Prepare and bake the yellow cake according to the box directions for a 9×13 inch pan. Let the cake cool for 15 minutes after removing it from the oven.
While the cake cools slightly, make the praline soak. Combine the sweetened condensed milk, heavy cream, brown sugar, and butter in a small saucepan. Heat over medium heat, stirring constantly until the butter melts and the mixture just begins to simmer. Remove from heat and stir in the 1/2 cup toasted pecans and vanilla extract. Set aside.
Use the handle of a wooden spoon or a large fork to poke holes about 1 inch apart all over the top of the warm cake.
Slowly pour the warm praline soak evenly over the entire surface of the cake, allowing the liquid to seep into the holes. Let the cake cool completely at room temperature, about 1 hour.
Prepare the pudding layer. Whisk the instant butterscotch pudding mix with 1 cup of cold milk until thickened, about 2 minutes. Gently fold the whipped topping into the pudding mixture until just combined.
Spread the pudding mixture evenly over the cooled cake.
Chill the cake in the refrigerator for at least 4 hours, or preferably overnight, to allow the soak to fully penetrate.
Before serving, sprinkle the remaining 1/4 cup chopped pecans over the top. Slice and serve cold.
Notes
Toast the pecans lightly in a dry skillet over medium heat for 3 to 5 minutes until fragrant before using them in the soak.
For the moistest result, do not overbake the yellow cake base.
You can substitute vanilla pudding mix for butterscotch if you prefer a less intense caramel flavor.