Preheat oven to 350 degrees F (175 degrees C). Grease a 9x9-inch baking pan.
Toss walnuts and chocolate chips with 1 teaspoon cocoa powder in a bowl until well coated; set aside. Mix tapioca flour, sorghum flour, teff flour, 1/3 cup cocoa powder, xanthan gum, and salt together in a separate bowl.
Beat butter and sugar together in a separate large bowl until creamy. Stir in eggs and vanilla extract. Gradually add flour mixture to butter mixture until fully incorporated. Fold in nuts and chocolate chips. Spread batter into the prepared baking pan.
Bake in the preheated oven until the top is dry and the edges have started to pull away from the sides of the pan, about 25 minutes.
