Wash beans and drain. Place in a large saucepan, and add cold water to cover; let soak overnight. Or place beans in a medium stockpot, and add water to cover; place over medium heat, and bring to a boil; boil 2 minutes, then let stand 1 hour.Advertisement
Drain beans and return to stockpot. Add water to cover by 2 inches. Place over medium heat, and bring to a boil, then reduce to a simmer. Cook beans until tender, about 30 minutes. Strain.
Working in batches, puree the cooked beans in a food processor until smooth. Set aside.
Combine the water and sugar in the stockpot. Place over high heat, and bring to a boil; boil, stirring frequently, until sugar dissolves completely.
Reduce heat to medium, and add half the bean paste to the simmering syrup. Stir slowly to incorporate, and continue cooking until the mixture comes to a boil, stirring constantly. Then add remaining bean paste, and continue stirring until mixture is soft and creamy, 3 to 5 minutes more. Remove from heat, and strain bean paste through a coarse sieve.
Scoop the bean paste onto a rimmed baking sheet; let cool to room temperature before using in frosting.
Shiro-an can be refrigerated airtight for several days. Bring to room temperature before using.