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

Add in one large egg white and mix until fully combined.

Pour in ¼ teaspoon of vanilla extract and mix briefly.

Whisk together 60g of sifted all-purpose flour, 15g of sifted cornstarch, and a pinch of salt in a separate bowl.

Gradually add the dry mixture to the wet ingredients while mixing on low speed until just combined.

Transfer your cookie batter into a piping bag fitted with a small round tip.

Line two baking sheets with parchment paper and pipe small oval shapes, leaving space between each.

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

Allow the cookies to cool on the baking sheets before transferring them to a wire rack.

Chop 200g of white chocolate and place it in a heatproof bowl.

Heat 50ml of heavy cream in a small saucepan until it simmers, then pour over the chopped chocolate.

Let the mixture sit for 1 minute, then stir until smooth.

Add 15g of unsalted butter to the ganache and stir until fully incorporated.

Allow the ganache to cool at room temperature for about 30 minutes.

Spread ganache between pairs of cookies and chill in the refrigerator for 30 minutes to set.

Serve your Shiroi Koibito at room temperature or chilled.