In a large mixing bowl, cream together butter and sugar until smooth.
Beat in the egg and vanilla. Set aside.
In a small bowl, stir together flour, baking soda, and baking powder.
Gradually blend the dry ingredients into the wet ingredients.
Add just enough of the buttermilk to moisten the dough and make it soft, but not wet. We used 1 tablespoon.
Roll dough into 1 inch balls and place on an ungreased cookie sheet. Slightly flatten the top of each cookie with the bottom of a glass that you've dipped in sugar.
With a pastry brush, moisten the top of each cookie with the remaining buttermilk.
Sprinkle with colored sugar or colored sprinkles.
Bake for 8 minutes. Let stand for 2 minutes on the baking sheets before removing to cool on a rack.