Line the cake tin with parchment paper. Bring water to a boil in a steamer. Dissolve food coloring in 1/4 teaspoon water, and set aside.
Stir the milk into the white bean paste, then add half of the sugar and the egg yolks.
Sift in the ingredients marked ✿ and mix.
In a separate bowl, beat the egg whites with a little sugar, add the rest of the sugar and continue beating until soft peaks form. No need to beat for too long.
Fold the egg whites, one-third at a time, into the batter in Step 3.
Add the dissolved food coloring, and stir lightly.
Pour the batter into the cake tin. Arrange the preserved cherry blossoms on top.
Put the tin in the steamer and cook for 10 minutes over high heat, then turn down the heat to low, and steam for 15 minutes.
After steaming, place a dampened, but tightly wrung out kitchen towel over the tin, until it's completely cooled. Even if it looks puffy, it will be just right after it cools.
If it's still high in the middle after cooling, invert it onto a piece of parchment paper for about 5 minutes. Don't use plastic wrap, or else it will stick.
Cut and serve.You could also cook it in a small pound cake tin or muffin cups.
It's really delicious if you chill it before serving.
Story Behind this Recipe
I learned how to make "Ukishima" from a Japanese confectionery chef. We made proper, traditional types with wasanbon (refined Japanese sugar), matcha and ama-natto (sugared beans). I made an unorthodox version by adding milk and making a marbled pattern!
If you beat the egg whites too stiff, or steam it over high heat for too long, the top will crack, like a pound cake. But don't worry, it'll still taste good. If you're using homemade shiro-an, or soft bean paste, add only 1 tablespoon of milk.