To make brown butter, just toss it in a sauce pan, and bring it to a boil. After a few minutes of boiling, the milk solids in the butter are going to toast and turn brown. Remove from the heat and pour into a liquid measuring cup to stop the cooking. Give it a stir and add 2-3 tablespoons of cold water to the butter to bring it back up to 1 cup of liquid. Set aside to cool to room temperature. This butter will give a rich, nutty flavor to the cookies.
Mix dry ingredients: flour, baking soda, salt and sift.
In a large bowl, add the sugars, vanilla, espresso powder, and cooled brown butter. Cream together with an electric hand mixer until light and fluffy, 1-2 minutes. Add the egg and yolk and beat until incorporated.
Add the dry ingredients, about of the mixture at a time, and beat between additions until just incorporated. Its okay for there to be a bit of unmixed flour on the edge of bowl, this will be incorporated in the next step.
Using a spatula to fold the chocolate chips and chunks into the dough
Scoop the cookies onto a parchment paper-lined baking sheet. For optimal flavor, cover and refrigerate for 1h or better overnight like I did.
Bake in a preheated oven 180oC for 12-14 minutes.
Let them sit on the baking pan for 2 minutes before removing to cooling rack.
