Preheat your oven to 350 degrees. Line a 9x13" baking dish with parchment paper.
For the crust, stir together the flour, brown sugar, and salt in a small bowl. Melt the butter and stir into the dry ingredients.
Press the crust into the pan and bake at 350° for 15 minutes. Cool for 10 minutes while you prepare the filling.
For the filling, whisk together the eggs, sweetened condensed milk, flour, brown sugar, melted butter, vanilla, and salt. Stir in 3 cups of the coconut flakes.
Pour the filling over the warm crust. Sprinkle the last cup of coconut evenly on top of the filling.
Turn the heat down to 325°, and bake for another 30-35 minutes, or until set in the middle and golden brown around the edges.
Let cool completely before cutting into squares.