These really are the BEST lemon bars I've ever had. I'm not sure if it's the buttery crust or creamy, tart lemon filling but they are absolutely killer and everyone that tries them agrees they are the BEST!
Preheat oven to 350 degrees and grease a 9x13" baking dish.
Combine cake mix with melted butter and egg and evenly press into bottom of the greased pan.
Using an electric mixer, combine cream cheese with eggs and mix until smooth. Add in vanilla and lemon juice, mixing until smooth once again, scraping the bowl as necessary. Repeat with the powdered sugar and butter, scrape bowl and mix until completely smooth. Pour filling over cake mix crust.
In a small bowl, combine sugars, coconut, flour and melted butter. Stir with a fork until combined and sprinkle evenly over the filling.
Bake at 350 degrees for 35-40 minutes or until crust is golden and middle is lightly set. The middle will still be jiggly like if you were to wiggle a jello mold. Allow to cool at room temperature 30 minutes and then refrigerate at least 2 hours before cutting.