Preheat oven to 350 °F (175 °C). Lightly spray a 9‑inch pie pan with cooking spray.
In a medium bowl, combine chopped pecans, softened butter, light brown sugar, and salt (if using). Mix until the mixture holds together when pressed.
Press the pecan mixture evenly into the bottom and up the sides of the prepared pie pan to form the crust.
Bake the crust for about 10–12 minutes, or until lightly toasted and fragrant. Let it cool.
In a medium saucepan, whisk together brown sugar, cornstarch, flour and salt. Gradually stir in the sweetened condensed milk + water mixture.
Cook over medium heat, stirring constantly, until the mixture thickens and reaches a gentle boil (about 5–7 minutes). Remove from heat and stir in butter until smooth.
Allow the mixture to cool slightly, then whisk in the beaten egg yolks and vanilla extract quickly and smoothly.
Pour the filling into the baked crust and smooth the top.
Chill the pie in the refrigerator for at least 4 hours or overnight until set.
Before serving, top with whipped cream as desired, slice and enjoy.