In a mixing bowl, beat 125g of softened unsalted butter with 80g of sifted powdered sugar using an electric mixer until light and fluffy.

Add one large egg white and mix on low speed until fully combined.

Add ¼ teaspoon of vanilla extract and mix briefly until incorporated.

In a separate bowl, whisk together 60g of all-purpose flour, 15g of cornstarch, and a pinch of salt. Gradually add to wet mixture, mixing gently until just combined.

Transfer cookie dough into a piping bag and pipe oval shapes onto lined baking sheets, spacing about 1 inch apart.

Preheat oven to 325°F (160°C) and bake the piped cookies for 8-12 minutes.

Once baked, remove from oven and let cookies cool on baking sheets for about 5 minutes, then transfer to a wire rack.

Heat 50ml of heavy cream until it simmers. Pour the hot cream over 200g of finely chopped white chocolate and let sit for a minute.

Mix the ganache until smooth and let it cool at room temperature for 10-15 minutes until spreadable.

Spread a layer of ganache on the flat side of one cookie and sandwich with another cookie.

Chill the assembled cookies for about 30 minutes for enhanced texture, then serve.

Enjoy your Shiroi Koibito cookies with a warm cup of green tea or as a delightful dessert.