Preheat your oven to 325°F (160°C). In a large mixing bowl, beat 1 cup of softened unsalted butter and 1 cup of sifted powdered sugar together on medium speed for about 3-5 minutes until light and fluffy.

Add the large egg white at room temperature and mix on low speed for 1-2 minutes until fully incorporated.

Pour in 1 teaspoon of vanilla extract and mix on low speed for about 30 seconds.

In a separate bowl, sift together 2 cups of all-purpose flour, 2 tablespoons of cornstarch, and a pinch of salt. Gradually add this to the wet ingredients, mixing on low until just combined.

Transfer the batter to a piping bag fitted with a small round tip.

Pipe oval shapes on lined baking sheets, leaving space between each cookie.

Bake for 8-12 minutes, until edges are lightly golden. Allow them to cool for 5 minutes before transferring to a wire rack.

In a small saucepan, heat 1 cup of heavy cream over medium heat until it just simmers. Pour it over 8 ounces of finely chopped white chocolate and let sit for a few minutes.

Stir the mixture until smooth and incorporate 2 tablespoons of unsalted butter. Let it cool until slightly thickened, about 15-20 minutes.

Spread a generous layer of ganache on one cookie and press another cookie on top. Repeat with remaining cookies.

For a firmer ganache, refrigerate for about 30 minutes before serving.

Enjoy your homemade Shiroi Koibito cookies at room temperature or chilled.