Preheat oven to 350F°. Line a 13x9-in. baking pan with parchment, leaving a 2 inch overhang. Spray bottom and sides with cooking spray.
In a medium bowl use a whisk to sift together the flour, brown sugar and salt. Once combined, add the melted butter stirring until fully moistened.
Press (or spread) the crust mixture onto the bottom of the prepared pan. Bake until light brown and set, around 15 minutes. Cool for 5 minutes on a wire rack. Reduce the oven setting to 325°F.
In a large bowl, whisk together the sweetened condensed milk, flour, brown sugar, eggs, vanilla extract coconut extract and salt until blended.
Stir in 3 cups of flaked coconut until evenly distributed. Pour the coconut mixture over the crust then sprinkle with remaining 1 cup coconut. (It's fine to add more than 1 cup to the top!)
Bake until the top is light golden brown, around 40 minutes. The filling should be set when the pan is gently shaken. Cool completely in the pan on a wire rack.
Lift the coconut bars from the pan using the parchment paper. Use a sharp knife to cut into bars.
Store in an airtight container chilled in the fridge for up to one week.