Combine the sugar, baking soda, salt, and buttermilk in a 4-quart saucepan (the soda will cause the mixture to foam excessively).
Cook over high heat, stirring constantly, to 210°.
Add the butter and 2 cups of the coconut.
Continue cooking and stirring over medium heat until 230°, or until a little of the mixture dropped into cold water forms a very soft ball.
Remove from the heat and cool 1 to 2 minutes; then beat with a spoon until the mixture looks cloudy and slightly thickened.
Immediately drop teaspoonfuls onto a lightly greased cookie sheet.
Before they set, top with the remaining 1/2 cup coconut.