Combine almond flour and matcha powder in a bowl.
Whip room-temperature butter and erythritol until creamy.
Add the egg and mix well.
Sift the dry ingredients and add them to the butter mixture; knead to form dough.
Chop dark chocolate and walnuts, mix them into the dough.
Rest the dough in the refrigerator for 30 minutes.
Shape the dough into balls and place on a baking sheet.
Bake in a preheated oven at 180°C (356°F) for 12–15 minutes.
Let cool before serving.