Step 1: Brown the butter in a medium saucepan over medium heat until it smells nutty and golden-brown bits appear; let it cool for 10 minutes.
Step 2: In a large bowl, whisk the browned butter with dark brown sugar and granulated sugar until well combined and smooth.
Step 3: Add the egg, egg yolk, and vanilla extract to the butter mixture, whisking until the batter looks glossy.
Step 4: Sift the flour, salt, and baking powder into the wet ingredients, then fold gently with a spatula until just combined.
Step 5: Fold in the chocolate chunks or your preferred mix-ins, then chill the dough in the refrigerator for at least 30 minutes.
Step 6: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Step 7: Scoop rounded tablespoons of dough onto the sheets and bake for 10-12 minutes until the edges are golden but the centers are still soft.
Step 8: Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.