Step 1

Sift the flour, baking powder, baking soda and kosher salt into a large bowl.

Step 2

Beat the butter, brown sugar and granulated sugar in a large bowl with a handheld electric mixer on medium-high speed until pale and fluffy, about 4 minutes, scraping down the sides of the bowl as needed. (Alternatively, use a stand mixer fitted with the paddle attachment.)

Step 3

Beat in the egg until combined, then beat in the vanilla. Reduce the mixer speed to low. Add the flour mixture and beat until combined. Fold in the white chocolate chips with a rubber spatula.

Step 4

Press plastic wrap directly onto the surface of the dough and refrigerate for at least 1 hour or preferably overnight.

Step 5

Position racks in the top and bottom thirds of the oven and preheat to 375 degrees F.

Step 6

Use a 1-ounce scoop to scoop the dough onto two ungreased baking sheets about 2 inches apart. Sprinkle the cookie with the flaky sea salt.

Step 7

Bake until the cookies are golden around the edges, but still soft in the middle, 12 to 15 minutes, rotating the pans halfway through from top to bottom and front to back. Let the cookies cool on the baking sheets for 10 minutes, then transfer them to a wire rack to cool completely, about 30 minutes.