Peel chestnuts and sweet potatoes. Boil chestnuts in water for 20 minutes or until tender, drain. Boil water and add 1/2 teaspoon salt and boil sweet potatoes for 10 minutes or until tender, drain.

In a saucepan crush potatoes and chestnuts until paste-like and add 1 cup sugar. Simmer for 5 minutes, constantly stirring. Add mirin, 1 cup sugar, 1/2 teaspoon salt and water and simmer 15 minutes or until like paste, constantly stirring to avoid burning.