Step 1

Heat ½ cup (1 stick) butter in a small saucepan over the lowest heat possible until melted (you don’t want it to sputter or brown), about 5 minutes.

Step 2

Meanwhile, whisk 1 cup (125 grams) all-purpose flour, ½ cup (63 grams) buckwheat flour, ½ tsp. baking powder, ½ tsp. baking soda, and 1¼ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt in a medium bowl.

Step 3

Coarsely chop 6 oz. bittersweet chocolate. Set aside a handful of chocolate in a small bowl.

Step 4

Scrape butter into a large bowl and add ⅔ cup (133 grams) light brown sugar and ½ cup (100 grams) granulated sugar. Whisk vigorously until butter has been absorbed into the sugar and no big lumps remain, about 30 seconds.

Step 5

Add 1 large egg, then 2 large egg yolks, one at a time, whisking until fully combined after each addition. Whisk in 1 tsp. vanilla extract. At this point, your mixture should look much lighter in color and be smooth, almost creamy.

Step 6

Add dry ingredients and use a rubber spatula or wooden spoon to stir until just incorporated and almost no dry streaks remain. Add chopped chocolate (but not the chocolate you reserved in the small bowl) to batter. Gently mix just to distribute. Cover bowl with an airtight bowl cover, a kitchen towel, or plastic wrap and chill 2 hours. (If you’re crunched for time, 1 hour will do, but cookies will be best after 2.)

Step 7

Place racks in upper and lower thirds of oven; preheat to 375°. Using a tablespoon measuring spoon, scoop out scant 2-Tbsp. portions of dough (or, if you have a scoop, this is a leveled-off #30 or a heaping #until you have 10 portions divided between 2 parchment-lined baking sheets (you want five per sheet—these will spread a bit!). Roll portions into balls and gently press a piece or 2 of reserved chocolate into each one. It’s okay to cram the chocolate on there—some pieces can even be vertical. Cover and chill any remaining dough.

Step 8

Bake cookies, rotating baking sheets top to bottom and front to back halfway through, until edges are golden brown and centers are puffed, 8–10 minutes. (Pull at 8 if you like your cookies softer and want to guarantee they’re still soft the next day!)

Step 9

Working one at a time, pull baking sheets out of the oven and tap lightly on the stove to deflate cookies. Sprinkle with Diamond Crystal kosher salt (if you’re using Morton, skip it: the crystals are too large). Let cookies cool on baking sheets 5 minutes, then transfer to a wire rack and let cool completely. Let baking sheets cool (to do this fast, run them under cold water), then turn parchment paper over. Repeat process with remaining dough, dividing evenly between baking sheets, to make 6–8 more cookies.

Step 10

Do Ahead: Cookies can be baked 3 days ahead. Let cool; store airtight at room temperature.