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

Incorporate 1 room-temperature large egg white into the mixture. Mix on low until fully combined for about 1-2 minutes.

Add a splash of vanilla extract if desired and mix for 30 seconds.

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

Transfer the dough into a piping bag fitted with a small round tip.

Pipe oval shapes of dough on a parchment-lined baking sheet.

Bake for 8-12 minutes until the edges turn lightly golden while the centers remain soft.

Let the cookies cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.

Chop 6 ounces of high-quality white chocolate and set aside. Heat ½ cup of heavy cream until just simmering, pour it over the chocolate and let sit for a minute before stirring until smooth. Incorporate 1 tablespoon of unsalted butter.

Allow the ganache to cool for 10-15 minutes until thickened.

Spread a layer of ganache on one cookie and top with another, pressing gently.

Optional: Chill the assembled cookies in the refrigerator for about 30 minutes for a firmer texture.