Steam or boil the purple sweet potato until soft. Peel and mash it smoothly.
In a bowl, mix mashed sweet potato, jaggery powder, and butter.
Add whole wheat flour, cocoa powder, and mix gently.
Pour in milk little by little to make a thick batter.
Grease a square tray and spread the batter evenly. Add the chopped nuts.
Steam for 20–25 minutes or bake at 180°C for 20 minutes.
Cool completely before cutting into squares.