Preheat oven to 350 degrees F (175 degrees C.) Spray 9 inch pie plate with non-stick spray. In a saucepan, melt butter and stir in sugar. Remove from stove. Stir in crushed pretzels until mixed well. Press into pie plate. Bake for 7 to 9 minutes. Cool before filling.
In a medium bowl, combine key lime juice and sweetened condensed milk. Mix well and pour into crust. Chill in refrigerator overnight.
When ready to serve. Spoon the whipped topping into a pastry bag and pipe it decoratively around the outer edge of filling.