In a small saucepan under medium heat, melt the butter and cook while stirring. The butter will first start to foam as it cooks and later take on a nutty aroma as the milk solids starts to brown. At this point, remove the browned butter from the heat. Transfer into a large mixing bowl to cool to room temperature, about 6-8 minutes.
In a small mixing bowl, whisk to combine the flour, baking soda, and salt. Set aside.
In the bowl with cooled browned butter, add and whisk to combine the brown sugar, granulated sugar, egg, egg yolk and vanilla extract.
Next, mix in the dry ingredients until a soft cookie dough forms. Fold in dark chocolate chunks. Refrigerate for at least 30 minutes or 1-2 hours.
Preheat oven to 350F. Line a large baking sheet with parchment paper.
Using a 1.5 ounce (3 tablespoons) cookie scooper, scoop chilled cookie dough onto prepared baking sheet evenly spaced apart. Top with additional chunks of chocolate, optional.
Bake for 10-12 minutes or until cookies look golden brown with crisp edges.
Let the cookies rest on the baking sheet for 5 minutes. Sprinkle tops with flaky sea salt, optional. Serve warm or transfer to a wire rack to cool to room temperature before serving.