Whisk together buckwheat flour, corn starch, tapioca starch, baking soda, salt, and matcha in a medium bowl.
In a large bowl, beat softened butter and sugars until creamy. You can use an electric mixer or a hand whisk.
Add the egg, and vanilla extract and mix well.
Add the dry ingredients and mix well until you get homogeneous cookie dough.
Let's chill the dough in the fridge for one hour.
Preheat the oven to 350ºF. Line baking sheets with parchment paper or silicone mats.
Drop large teaspoons 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 chocolate chunks or chips on the surface of the cookies.
Bake for 12 minutes or until golden brown.
Let cool before removing.
Cookies can be stored in an airtight container at room temperature for up to 5 days.