Preheat & Prep. To start, preheat oven temperature to 350 degrees F. Line two cookie sheets with parchment paper and set aside. Melt Butter & Chocolate. In a microwave-safe bowl, combine butter and 8 ounces chocolate chips. Melt in 15-20 second intervals on low heat, stirring in between each interval, until the chocolate is smooth and glossy. Set aside to cool slightly.
Whip Eggs, Sugars & Vanilla. In a large mixing bowl, combine brown sugar, granulated sugar, eggs, and vanilla. Use a hand mixer to beat the eggs mixture 2-3 minutes on medium speed, or until much lighter in color, doubled in volume, and the sugar is dissolved almost completely. (When you rub this mixture between your fingers, there should be almost NO grit. This is a critical step, so don't rush it!)
Add Chocolate To Egg Mixture. Pour cooled melted chocolate mixture into the egg mixture and mix to combine.
Add Dry Ingredients. Then, add gluten-free flour, dark cocoa powder, baking powder, and salt to the wet ingredients, and mix until just combined. (At this point, the cookie dough should look and feel like thick brownie batter.)
Fold In Mini Chocolate Chips until evenly distributed in the cookie dough.
Scoop & Bake Cookies. Working in batches, use a medium cookie scoop to portion 8 cookies per baking sheet. (The dough balls should be 1.5 Tablespoons in size.) Make sure the cookies have plenty of room around them to spread! Bake cookies 9-11 minutes in the preheated oven, or until set on the outside and just a little shiny in the middle. (Mine are usually done at 10-10 1/2 minutes!) Repeat until all cookies are baked. Cool Cookies Completely. Let the cookies cool 1-2 minutes on the baking sheet before transferring to a wire rack to finish cooling completely. Enjoy once cooled & store leftover cookies in an airtight container at room temperature 2-3 days, or freeze up to 2 months.