First, preheat the oven to 375º, and make sure your butter is softened. Then, in a small bowl combine flour, baking soda, baking powder, and salt and set aside.
Next, in the stand mixer, cream the butter and sugar together on a medium speed until completely smooth. Scrape down the sides of the stand mixer and mix again to ensure it is well combined. This is an important step. If you skip this step, the cookies will not rise correctly in the oven.
Then, add the vanilla and egg into the stand mixer and mix on a medium side, scraping down the sides if needed until the eggs and vanilla are incorporated well. Slowly add in the dry ingredients, mixing until a dough ball forms.
Then, roll the dough into 36 dough balls. Next, place the dough balls on an ungreased cookie sheet 4 inches apart, and bake in the oven for 8-10 minutes. They will still look pretty thick when you take them out of the oven.
Lastly, take the cookie sheet out of the oven and slam it on the counter 5 or 6 times, so that it will flatten out the cookies. Then, allow the cookies to cool completely on a cooling rack. If you want to add sprinkles or icing you can do this at the end. To add just sprinkles, baste each cookie with melted butter and place the sprinkles on, so that they stick to the melted butter.
Make sure you cream the butter and sugar completely. At each step, make sure to mix the dough very thoroughly, making sure to scrape down the sides each time. This will ensure that your cookies cook evenly.