Preheat your oven to 350°F (175°C). Combine the finely chopped pecans, softened unsalted butter, light brown sugar, and optional salt until the mixture resembles coarse crumbs. Press this mixture evenly into a 9-inch pie pan. Bake for 10-12 minutes or until golden brown. Let it cool completely.

In a medium saucepan, whisk together brown sugar, cornstarch, all-purpose flour, and a pinch of salt. Gradually pour in the sweetened condensed milk mixed with water, stirring continuously over medium heat until thickened and gently boiling (about 5-7 minutes). Remove from heat and stir in unsalted butter until smooth.

Let the filling cool slightly for 2-3 minutes before adding the beaten egg yolks and vanilla extract. Whisk quickly and pour the filling into the cooled pecan crust, smoothing the top evenly.

Cover the pie with plastic wrap or a lid and refrigerate for at least 4 hours to set the filling.

Once chilled, remove the pie from the refrigerator. Top with a layer of whipped cream and optionally sprinkle crushed pecans or brown sugar. Slice into wedges and serve.