Cream the softened unsalted butter and sifted powdered sugar together using an electric mixer on medium speed for about 3-5 minutes until light and fluffy.

Add the room-temperature egg white to the creamed mixture and mix until fully combined, about 1-2 minutes.

If desired, pour in the vanilla extract and mix briefly until incorporated.

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

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

Line two baking sheets with parchment paper and pipe small oval shapes of cookie batter onto the sheets, leaving space in between.

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

After baking, cool the cookies on the sheets for 5 minutes before transferring to a wire rack to cool completely.

Finely chop the white chocolate and place it in a heatproof bowl.

In a small saucepan, heat the heavy cream over medium heat until it simmers, about 2-3 minutes.

Pour the hot cream over the chopped chocolate and let it sit for 1 minute, then stir until smooth.

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

Let the ganache sit at room temperature for about 10-15 minutes until it thickens slightly.

Select pairs of cookies that are similar in size and shape. Spread ganache on the flat side of one cookie and press another on top.

Continue the assembly process with the remaining cookies and ganache.

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

Serve at room temperature or chilled.