To brown the butter:

1. Place the cup of butter in a medium sized saucepan and heat on medium heat for about 5 minutes or until the butter no longer makes noise (that’s the water disappearing).
2. Watch very closely as the milk solids will be getting brown and stuck to the bottom of the pan. Swirl the pan gently another minute or so until the butter is a deep, golden brown.
3. Remove from the heat, let cool for several minutes and then transfer to a glass jar and refrigerate until ready for use.

To make the cookies:

1. Pre-heat the oven to 375 degrees fahrenheit.
2. Combine the flour, baking soda, baking powder and salt in a bowl and set aside.
3. In the bowl of a stand mixer with the paddle attached, combine the butter and sugar.  Beat on medium speed for about a minute and is well combined.
4. Add the egg yolks and vanilla and beat until well combined.
5. Add in the flour mixture and beat just until combined.
6. Form the dough into balls about an inch in diameter and then roll in the sugar.
7. Bake the cookies on a baking sheet for 8 to 10 minutes or just until slightly brown.
8. They may seem slightly undercooked but that’s perfect. Resist letting them bake longer.  Immediately remove the cookies from the baking sheet onto a cooling rack and let cool completely.