Prepare Brown Butter: In a saucepan, melt butter over medium heat until golden brown, about 5-7 minutes. Cool in the fridge until solidified.

Preheat Oven: Set your oven to 400°F (200°C) and line baking sheets with parchment paper.

Mix Butter and Sugars: In a stand mixer, combine cooled brown butter with dark brown sugar and granulated sugar. Beat on medium until fluffy.

Add Wet Ingredients: Incorporate egg yolks, vanilla extract, and sour cream into the mixture. Beat until well combined.

Combine Dry Ingredients: Whisk together cake flour, all-purpose flour, baking powder, baking soda, and kosher salt. Gradually mix with wet ingredients.

Fold in Chocolate: Gently fold in the chocolate using a spatula until evenly distributed.

Portion and Shape: Roll dough into 2-tablespoon-sized balls and place on baking sheets, leaving space between them.

Sprinkle with Salt: Top dough balls with extra chocolate and a sprinkle of flaky sea salt.

Bake: Bake for 7-8 minutes, or until golden edges appear. Rotate trays halfway through baking.

Cool: Let cookies cool on the sheets for 15 minutes before transferring to wire racks.