Begin by creaming 1 cup of softened unsalted butter with 1 cup of sifted powdered sugar in a mixing bowl. Use an electric mixer on medium speed for about 3-5 minutes until the mixture is light, fluffy, and pale in color.

Add one room-temperature large egg white to the creamed mixture and mix on low speed for 1-2 minutes until well combined and smooth.

If desired, briefly mix in 1 teaspoon of vanilla extract to the batter for an extra layer of flavor.

In a separate bowl, whisk together 2 cups of sifted all-purpose flour, 2 tablespoons of cornstarch, and a pinch of salt. Gradually add this dry mixture to your wet ingredients, mixing on low speed until just combined.

Transfer the cookie batter into a piping bag fitted with a round tip. On prepared baking sheets lined with parchment paper, pipe uniform oval shapes, about 2 inches long.

Preheat your oven to 325°F (160°C). Place the baking sheets in the oven and bake the cookies for 8-12 minutes or until the edges are lightly golden.

Once baked, remove the cookies from the oven and let them cool on the sheets for about 5 minutes before transferring them to a wire rack.

Chop 5 ounces of high-quality white chocolate into small pieces and set aside. Heat 1/2 cup of heavy cream until it just begins to simmer, then pour over the chopped chocolate.

Once your ganache is smooth, stir in 1 tablespoon of unsalted butter until fully combined. Allow the ganache to cool for 10-15 minutes.

To assemble your Shiroi Koibito, spread a generous layer of ganache on the flat side of one cookie and press another cookie on top.

For an enhanced texture, consider refrigerating the assembled cookies for about 30 minutes.

Finally, serve your delicious Shiroi Koibito at room temperature or chilled.