Cut butter into large pieces of the same size, so that it melts evenly, and cook in a medium saucepan over medium-low heat. Don´t stop stirring until it browns, the process takes approximately 5 minutes. Pour the butter into a large bowl and let cool for about 5 minutes.
Whisk buckwheat flour, corn and tapioca starch, baking soda, salt, and cacao in a medium bowl.
Add the sugars to the butter and mix well. Add the eggs, one at a time, mixing well after each addition. Add the vanilla extract.
Stir the dry ingredients into the mixture and mix until just combined.
Finally, add 3/4 of the chocolate chunks.
Let dough chill for at least one hour.
Preheat oven to 350ºF.
Line baking sheets with parchment paper or silicone mats.
Drop large balls of cookie dough on the prepared sheets, spacing them an inch or so. Use your fingers to gently press down on the balls and flatten them slightly. Place the remaining chocolate chunks on the surface of the cookies.
Bake for 10 minutes.
Let cool before removing.