Cut butter into four equal chunks and place in a light colored, shallow pan. Heat pan over medium high heat and stir butter until it is light brown and smells a little like hazelnuts. It's important to watch it closely or it will burn, but browning the butter is what makes the dough so delicious. See this tutorial
if you're not quite sure what to look for. Allow butter to cool before moving on to the next step or your chocolate chips may melt a little. You can put it in the fridge if you're in a hurry.