Instructions

1. Position racks in the upper and lower thirds of the oven and preheat to 350 degrees Fahrenheit. Line 2 rimless cookie sheets with parchment paper.
2. Place the butter in a small, heavy-bottomed saucepan set over the lowest possible heat. Add 8 ounces of the chocolate (not all of it!) and melt together, stirring frequently to prevent the chocolate from scorching. Continue cooking until the mixture is pleasantly warm, but not super hot, to the touch. Remove from the heat and keep warm. Sift the buckwheat flour, tapioca flour and baking powder into a small bowl and set aside.
3. Meanwhile, place the eggs, sugar and salt in the bowl of a stand mixer fitted with the paddle attachment and whip on medium-high speed until the mixture is very light and fluffy, 5 minutes (or use a mixing bowl and hand mixer). Turn the mixer to low and stir in the vanilla until just combined, then the warm chocolate butter mixture. Add the flour mixture and beat on low speed until combined. Remove the bowl from the mixer and use a flexible silicone spatula to fold in the remaining 4 ounces chopped chocolate.
4. If the batter is very runny, let it cool for a few minutes until it firms to the consistency of a thick brownie batter. Use a #40 spring-loaded ice cream scoop or 2 spoons to drop heaping tablespoons of batter onto the prepared baking sheets, spacing them at least 2 inches apart. Top each cookie with a few chunks of chocolate and a few flecks of flaky salt.
5. Bake the cookies until puffed and cracked and the edges are set, 8–12 minutes, rotating the pans front to back and top to bottom halfway through baking. Let cool on the pans. Enjoy warm or at room temperature. The cookies are best the day of baking but will keep, airtight at room temperature, for up to 3 days.