• Melt all but two tablespoons of butter in a medium saucepan over medium heat. Cook, stirring frequently, until the butter foams and turns a deep golden brown with a nutty aroma.
• Remove from heat. Whisk the remaining butter into the hot brown butter until melted. Let the brown butter cool to room temperature. About 10-15 minutes.
• Preheat your oven to 375º Fahrenheit. Line two baking sheets with parchment paper.
• Whisk the flour, baking soda, and salt in a large bowl. Set aside.
• Combine the cooled brown butter, dark brown sugar, and granulated sugar in a separate large bowl. Mix until well blended, about 2-3 minutes. Add the egg, extra yolk, and vanilla extract. Mix until smooth, then add the sour cream.
• Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the semi-sweet chocolate chunks and milk chocolate chips.
• Scoop the dough into balls (about 2 tablespoons each) and place them onto the prepared baking sheets, spacing them about 2 inches apart.
• Bake for 10-12 minutes at 375º Fahrenheit, until the edges are golden but the centers are still soft. Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.

OPTIONAL

• Sprinkle flaky salt on top of the warm cookies for an optional extra touch of flavor.