I know what you're thinking. IMPOSSIBLE!!!

1/2 cup of mashed yam
2 tbsp of butter
1/2 cup of all purpose flour
1/4 cup of brown sugar
2 tbsp baking powder
1/4 tsp salt
2 tbsp unsweetened cocao powder

Method:
Mash your yam with melted butter and salt

Mix together then add baking powder and brown sugar and mix

Then add cocao powder and flour and mix till a dough is formed

Roll it out and cut out the cookies to your desired size

Preheat the oven to 350°F, place the cookies in a greased pan and bake for 10 to 12 minutes

See I told it was possible