In a small saucepan over medium-high heat, brown the butter. Stir continuously with a silicone spatula as the butter melts, foams, and develops brown specks. Once the sizzling begins to subside and the butter is golden brown, transfer it to a bowl and let it cool to room temperature or slightly warm.
In a medium bowl, whisk together the flour, milk powder, baking soda, and kosher salt. Set aside.
In a large mixing bowl, whisk the cooled brown butter, brown sugar, and granulated sugar until combined. Add the white miso, vanilla extract, and egg, whisking until smooth.
Fold the dry ingredients into the wet ingredients using a rubber spatula until only a few streaks of flour remain. Add the chopped dark chocolate and fold until evenly distributed.
Cover the dough and refrigerate for at least 1 hour or up to 24 hours.
Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
Scoop the dough into 2-tablespoon portions and place them on the prepared baking sheets, spacing them at least 2 inches apart.
Bake for 12 minutes, rotating and switching the pans halfway through baking.
Remove the pans from the oven and carefully drop them from a few inches above the counter to create rippled cookie tops.
Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.