Preheat oven to 325 degrees F (165 degrees C). Spray a baking sheet with cooking spray.
Stir buckwheat flour, white sugar, brown rice flour, potato flour, lavender, vanilla powder, and baking powder together in a large bowl, making a well in the center. Whisk eggs, canola oil, and egg white together in a small bowl until well blended. Pour egg mixture into well and stir until dough is combined.
Divide dough in half; form 2 logs with hands. Transfer logs to the prepared baking sheet.
Bake in the preheated oven for 30 minutes. Remove from oven; reduce temperature to 300 degrees F (150 degrees C). Slicing gently to avoid crumbling, cut logs into 1/2- to 3/4-inch cookies with a sharp knife. Lay cookies on sides on the baking sheet.
Bake in the preheated oven for 10 minutes. Turn cookies over and continue baking until hardened, about 10 minutes more. Remove from oven; cool.