Cream butter and sugar together until light and fluffy. Add baking soda, and all flour at once, stirring in as much as you can and then kneading in the rest.
Form into balls the size of a large marble. Place on a parchment paper-covered cookie sheet and flatten slightly with your palm. Alternatively, press dough into a 9x13" baking pan.
Preheat oven to 400 degrees, with the rack in the middle slot. When heated, put cookie sheet in the oven and immediately lower to 300. Bake between 10-20 minutes, depending on thickness, watching carefully and removing as soon as edges just begin to brown.
If desired, cool and frost; combine remaining ingredients until smooth and spread over cookies. Decorate as desired.