Combine the flour, cocoa powder, baking powder, and kosher salt together into a bowl or onto a sheet of parchment. Set aside.
Melt the chocolate in a microwave-safe bowl. Stir after each 15-second interval. Continue until the chocolate is fully melted.
In a stand mixer fitted with the paddle attachment, cream the butter with the brown sugar and granulated sugar until just combined. Do not overcream.
Add the eggs one at a time, beating after each addition.
Add the vanilla extract and mix to combine.
Pour in the melted, cooled dark chocolate and beat until incorporated.
Add the dry ingredients and mix on low until the flour just disappears, then fold in the chocolate chips.
Scoop the dough with a large cookie scoop onto a lined baking sheet and flatten slightly. Sprinkle with Maldon salt if using.
Chill the scooped dough for at least 30 minutes, or refrigerate the dough tightly wrapped for up to one week, or freeze for up to two months.
Bake at 350°F conventional for 7 to 8 minutes, until the edges are set and the tops are fully matte with small cracks across the surface. The centers will look slightly underdone. Let cool on the pan.
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Notes
Flour: Weighing the flour on a kitchen scale is the most reliable method. Too much flour produces dry, cakey cookies. If measured by volume, fluff the flour, spoon it into the cup, and level with a straight edge. Doneness cue: Look for tops that are fully matte with small cracks across the surface and edges that appear set. The centers will still look slightly soft — pull them at that point and let carry-over heat finish the job on the pan. Chilling: Chill scooped dough for at least 30 minutes before baking for the thickest, chewiest result. Dough can be refrigerated for up to one week or frozen for up to two months. Storage: Baked cookies keep in an airtight container at room temperature for up to 10 days, or frozen for up to 2 months.