Peel and steam the sweet potatoes.
Mash the steamed sweet potatoes and let them cool.
Melt dark chocolate in a double boiler.
Mix the melted chocolate and egg into the mashed sweet potatoes.
Add cocoa powder, baking powder, and salt, and stir well.
Add nuts and mix evenly.
Pour the batter into a pan and spread it evenly.
Bake at 180°C for 25-30 minutes.
Let it cool and cut into bite-size pieces.