Add the melted butter to a medium bowl and whisk in the brown sugar, almond butter, miso, and vanilla until smooth. Whisk in the egg until smooth.

In another medium bowl, stir together the oat flour, tapioca starch, and baking soda. Add the flour mixture to the butter mixture and stir together using a wooden spoon until no dry flour remains and a soft dough has formed. Add the chocolate and stir until evenly distributed. Cover the bowl with plastic wrap and refrigerate for at least 4 hours or preferably overnight.

Set one oven rack in the middle position and a second rack in the bottom position and heat the oven to 350°F. Line two rimmed baking sheets with parchment paper.

For each cookie, use a 1-tablespoon measuring spoon to scoop 3-tablespoon portions of dough and use your hands to shape the dough into a ball. Place six balls of dough on one parchment-lined baking sheet and seven balls on the other, spacing them 3 inches apart. Bake the cookies until golden brown, 12 to 13 minutes.

Use oven mitts to remove the baking sheets from the oven and place on the stovetop or cooling racks. Let the cookies cool on the baking sheets for 10 minutes before using a spatula to transfer them to a serving plate. Enjoy warm or let them fully cool on the plate for about 30 minutes before eating. The cookies will keep in an airtight container at room temperature for up to 3 days.