Preheat oven to 350 degrees. Combine butter and flour with a pastry blender to make course crumbs. Stir in pecans. Pat mixture into an ungreased 9-x13-inch cake pan. Bake 10–15 minutes or until very light brown. Cool. Beat cream cheese with Splenda until no lumps remain. Beat in choice of extract (vanilla or almond is excellent with a raspberry topping; Crème de Menthe is good with chocolate pudding, etc.). Fold in 2 cups Cool Whip. Spread over crust. For topping, whisk together instant pudding and milk until thick. Carefully pour pudding, pie filling, or fresh fruit over cream cheese layer. Chill for 30 minutes, then spread with remaining Cool Whip. Chill 1 hour.