Preheat oven to 375ºF. Line baking sheets with parchment paper or nonstick mats.
In a large bowl, beat butter and sugar with an electric mixer for 2-3 minutes. Add egg and vanilla. Mix well.
In a small bowl, whisk flour, baking soda, and baking powder.
Gradually mix dry ingredients into wet ingredients. Dough will be crumbly.
Add buttermilk 1 tablespoon at a time until dough is soft but not wet.
Roll dough into 1-inch balls and place on baking sheets. Flatten with a sugar-dipped glass.
Brush tops with remaining buttermilk and sprinkle with colored sugar or sprinkles.
Bake for 8 minutes, until just set. Let cool on baking sheets for 2 minutes, then transfer to a wire rack to cool completely.