Preheat your oven to 350 degrees F. Line a 9-inch x 13-inch baking pan with parchment paper or aluminum foil and set aside.
Add the melted butter and brown sugar to a large mixing bowl and mix together. Then add in flour and salt, and mix until a dough forms. Press the dough into the bottom of the prepared pan in an even layer.
Bake for 12-15 minutes, until lightly golden brown. Remove from the oven and set aside to cool for 5-10 minutes. Reduce the oven temperature to 325 degrees F.
Filling
To a large mixing bowl, add the eggs, sweetened condensed milk, flour, brown sugar, melted butter, vanilla, and salt. Whisk everything together. Stir in 3 cups of the shredded coconut.
Pour the filling over the cooled crust and spread it out evenly. Then sprinkle the remaining 1 cup of shredded coconut over the filling.
Bake for 25-30 minutes, or until set and the coconut on top is lightly golden brown. Remove from the oven and cool to room temperature.
Slice into 20 bars using a 4x5 grid pattern and serve. Store any leftovers in an airtight container in the fridge.
Notes
Recipe yield will vary based on how small you cut the bars.