Preheat oven to 350°. Line a 13x9-in. baking pan with parchment paper, spraying only the sides, lightly.
In a large bowl add the flour, brown sugar and salt, mix well. Add the 1 cup melted butter and mix until well combined. Press onto bottom of prepared pan. Bake until light brown, about 15 minutes. Cool 10 minutes on a wire rack. Reduce oven setting to 325°.
In a large bowl combine the eggs, sweetened condensed milk, flour, brown sugar, 1/4 cup melted butter, vanilla and salt, whisk until blended. Stir in 3 cups of the coconut and spread the mixture over the warm crust. Sprinkle with the remaining 1 cup coconut. Bake until light golden brown, 25-30 minutes.
Allow bars to cool on a wire rack. Lifting edges of parchment paper, remove from pan. Cut into bars.