Steps
1. Wash, peel and boil the root crop tubers.
2. Mix the sugar, starch, salt and cocoa powder together in a bowl.
3. Slowly stir in the milk.
4. Place the bowl over a saucepan of boiling water and stir until the mixture has thickened.
5. Separate the egg yolk from the egg white.
6. Beat the egg yolk well. Add a little of the hot milk mixture to the beaten egg yolk.
7. Mix the egg yolk with the remainder of the milk mixture.
8. Place the bowl back over the saucepan of hot water. Continue to stir for 4 - 5 minutes.
9. Remove the bowl from the saucepan of hot water and allow the contents to cool.
10. Mix the cold water together with the mashed root crop and lime juice. Press the mixture through a sieve.
11. Combine the root crop mixture with the milk mixture and place in the bread tin or plastic container.
12. Freeze the mixture for one hour.
13. Take the mixture out of the container and beat well.
14. Return the mixture to the loaf tin or plastic container and continue to freeze until hard.

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