Layer 1: Preheat oven to 350 degrees. Melt butter. In a bowl, mix the butter, flour, and pecans. Spread in a 13" X 9" casserole dish and press down with your fingers making sure to cover the bottom of the dish. Bake for 25 minutes. Cool completely.
Layer 2: With a mixer, mix the cream cheese and sugar. Fold in Cool Whip and spread over the crust.
Layer 3: With a mixer, mix the sweetened condensed milk and lemon juice. Spread over the cream cheese layer.
Layer 4: Spread the rest of the Cool Whip over the top of the pie. Refrigerate for several hour or overnight before serving.