Begin by creaming 125g of softened unsalted butter with 80g of powdered sugar in a large mixing bowl using an electric mixer on medium speed for about 3-5 minutes until light and fluffy.

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

If desired, mix in 1/4 teaspoon of vanilla extract for added flavor, then continue mixing until just combined.

In a separate bowl, whisk together 60g of all-purpose flour, 15g of cornstarch, and a pinch of salt. Gradually add this mixture into the wet ingredients while mixing on low speed until just combined.

Prepare a piping bag fitted with a small round tip and fill it with the cookie batter.

Pipe small ovals of cookie dough onto a lined baking sheet, leaving about 1 inch of space between each.

Preheat your oven to 325°F (160°C). Bake the cookies for approximately 8-12 minutes until lightly golden at the edges.

Let the cookies rest for about 5 minutes before transferring them to a wire rack to cool completely.

To make the ganache, heat 50ml of heavy cream until it simmers. Pour it over 200g of chopped white chocolate in a bowl and stir until smooth.

Allow the ganache to cool at room temperature for about 10-15 minutes until thickened.

Spread a layer of ganache on the flat side of one cookie and press another cookie on top to form a sandwich.

Chill the assembled cookies in the refrigerator for about 30 minutes if you prefer a firmer texture.

Enjoy the Shiroi Koibito cookies at room temperature or chilled.