Steam or boil the sweet potato, peel, and mash it.
Melt the butter and chocolate together over a double boiler.
Mix the mashed sweet potato with the melted butter and chocolate, sugar, and eggs.
Sift cake flour and baking powder into the batter and stir to combine.
Pour the batter into a madeleine pan.
Bake in a preheated oven at 180°C (356°F) for 15~20 minutes.