Step 1

Adjust an oven rack to the middle of the oven. Preheat the oven to 400F [200C]. Line two sheet pans with parchment paper.

Step 2

In a medium bowl, whisk together the flour, baking powder, and baking soda.

Step 3

In the bowl of a stand mixer fitted with a paddle, beat the butter on medium speed until creamy, about 1 minute. Add the granulated and brown sugars and salt and beat on medium speed until light and fluffy, 2 to 3 minutes. Scrape down the sides of the bowl, add the egg, yolk, and vanilla, and mix until smooth. Add the flour mixture and beat on low speed until just combined. Add the chocolate and mix into the batter on low speed. Use a rubber spatula to give the dough a final mix and make sure it is completely combined.

Step 4

Form the dough into balls, about 2 tablespoons each [50 g], and place 8 cookies on each sheet pan. Sprinkle each cookie with a little flaky salt, if desired.

Step 5

Bake one pan at a time, rotating halfway through baking. Bake the cookies until the tops are golden brown and the cookies are slightly puffed and starting to crinkle, 10 to 11 minutes. Remove the baking sheet from the oven and use the back of a spatula to gently press the top of each cookie to flatten it. Let the cookies rest on the sheet pan for 5 minutes, then transfer them to a wire rack to finish cooling. Cookies are best slightly warm but can be stored in an airtight container at room temperature for up to three days.

Step 6

Cookies will soften the longer they sit. Cookie dough can be formed and refrigerated overnight, then baked the next day; the cookies may need an extra minute if chilled. Cookie dough can also be formed into balls and frozen for up to 2 weeks; let the cookies sit out at room temperature until the oven preheats. The cookies will need an extra minute if baked frozen.