Preheat the Oven: Preheat your oven to 350°F (175°C). Spray a 9 x 13 rectangular pan with baking spray and set aside.
Prepare the Crust: In a large bowl, mix together the yellow cake mix, 1 egg, and melted butter. Stir until well combined, then pat the mixture evenly into the bottom of the prepared pan. Set aside.
Make the Filling: In another large bowl, beat the softened cream cheese until smooth. Add the eggs, one at a time, and mix well. Stir in the vanilla extract, coconut extract, fresh lemon juice, powdered sugar, and melted butter. Mix until fully incorporated.
Assemble the Layers: Pour the filling over the crust in the pan, spreading it evenly.
Prepare the Topping: In a bowl, combine the brown sugar, granulated sugar, and sweetened coconut. Stir in the melted butter until crumbly and then add the flour. Mix until it resembles coarse crumbs.
Bake: Sprinkle the crumb topping over the filling. Bake in the preheated oven for 35-40 minutes, keeping an eye on it to avoid overbaking. It should be set in the center but still sticky.
Cool and Serve: Once baked, allow the bars to cool completely in the pan before cutting them into squares. Enjoy your delicious Lemon Coconut Sticky Butter Bars!