Steps
1. Make the pastry by mixing the flour, salt and baking powder together.
2. Add the margarine and rub into the flour using the fingertips. Mix until the flour mixture looks like breadcrumbs.
3. Add cold water a little at a time and mix well. Continue to add a little bit of water at a time until the flour mixture forms a smooth dough.
4. Roll the dough out until it is large enough to line the inside of a shallow pudding basin
5. Wash, peel, boil and mash the sweet potato.
6. Press the sweet potato through a sieve.
7. Mix the sieved sweet potato with the milk, sugar and well beaten eggs.
8. Stir in the cinnamon, salt, lemon juice.
9. Pour the filling mixture into the pastry lined shallow pudding bowl.
10. Cook the pie in a hot oven (230?C) for 10 minutes then reduce to a medium heat (160?C) and cook for a further 15 minutes until the filling puffs up a little and the blade of an inserted knife comes out clean.
11. Melt the chocolate. with the butter in a saucepan. Remove from the heat and add I or 2 tablespoons of boiling water. Beat the mixture until smooth and pour over the warm pie and chill well before serving.

Source
Food and Agriculture Organization of the United Nations (1994), Utilizing root crops, RAPA Publication 1994/30, Regional Office for Asia & the Pacific Bangkok, Thailand