Preheat the oven to 350F (180C)
Add the butter to a saucepan. Cook on medium heat, stirring occasionally. The butter will start changing color, becoming dark amber. The milk solids will fall to the bottom of the pan. As soon as they start turning dark, remove the butter from the heat. This will take about 5 minutes. Let it cool while preparing the other ingredients.
In a bowl, whisk the flour, oats, baking soda, salt, and cinnamon. Set aside.
In another bowl, whisk the brown butter, brown sugar, and white sugar. Then add the egg and egg yolk, plus the vanilla. Mix until smooth.
Fold in the dry ingredients to the wet mixture.
Add the chocolate chips. Mix them into the batter.
Line a baking tray with parchment paper.
Using a cookie scooper or a spoon, make cookie balls the size of a golf ball. Place them on the baking tray, leaving about 2 inches of space between each cookie. Press the cookies slightly.
Bake the cookies on the middle rack for 12-15 minutes, or until golden brown at the edges.
Take them out of the oven and let them cool for 2-3 minutes before transferring them to a cooling rack. Enjoy!