In a bowl, combine cracker crumbs and butter. Press into an ungreased 9-in. square dish. Refrigerate until firm.
In a bowl, combine gelatin and water; stir until dissolved. Cool to room temperature. In a blender, combine the cottage cheese and sugar substitute; cover and process until smooth. Slowly add gelatin mixture and blend until smooth. Pour into crust. Refrigerate until firm.
Meanwhile, in a saucepan, combine cornstarch and water until smooth. Add pineapple. Bring to a boil. Cook and stir for 2 minutes or until thickened. Cool to room temperature. Spread over gelatin layer. Refrigerate for at least 1 hours before serving.