Trust us: Adding a bit of salt to sweet treats makes them taste even better.
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line both rimmed baking sheets with parchment paper.
In medium bowl, whisk together flour, baking soda, and baking powder.
In large bowl, whisk dark brown sugar and melted butter until smooth and no lumps remain, about 30 seconds. Add whole egg, egg yolk, and vanilla and whisk until well combined, about 30 seconds.
Add flour mixture to sugar mixture and use rubber spatula to stir until just combined and no dry flour is visible, about 1 minute.
Use your slightly wet hands to roll dough into 20 balls (about 2 tablespoons each). Place dough balls on parchment-lined baking sheets, leaving space between balls (there will be 10 balls on each baking sheet).
Place baking sheets in oven. Bake cookies for 6 minutes. Use oven mitts to rotate and switch positions of baking sheets in oven (ask an adult for help). Bake cookies until edges are beginning to set but centers are still soft and puffy (cookies will look raw between cracks and seem underdone), 6 to 8 more minutes.