Place the brown sugar and coconut oil in the bowl of a stand mixer fitted with the paddle attachment and beat on medium to high speed until thoroughly combined, about 1 minute. Add the milk, miso and vanilla and beat for 1 minute.
Whisk the flour and baking soda in a medium bowl. Slowly add the dry ingredients to the miso mixture and beat on medium speed until just combined. Add the chocolate chips and beat until combined.
Cover the dough and refrigerate for 1 hour.
Position racks in the upper and lower thirds of the oven and preheat to 350 degrees. Line each of 2 baking sheets with parchment or a silicone baking mat.
Scoop the dough with a 1-inch ice cream scoop (or tablespoon) and place 15 dough balls on each of the prepared baking sheets. Gently press the dough down into a circle with your palm or fingers.
Bake, rotating and switching the positions of the baking sheets halfway through, until the cookies are golden, 10 to 12 minutes. Let the cookies rest on the baking sheets for 1 minute then transfer to wire racks to cool.
Serve or store cookies in a tightly sealed container for up to 6 days.