In a large mixing bowl, beat the butter and powdered sugar together until light and fluffy, about 3-5 minutes. Add the egg white and vanilla extract, mixing until glossy.

In another bowl, whisk together all-purpose flour, cornstarch, and salt. Gradually fold into the wet ingredients, mixing until just combined.

Transfer the batter into a piping bag and pipe small oval shapes onto a parchment-lined baking sheet.

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

Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack.

Heat heavy cream on medium until simmering. Pour over chopped white chocolate, let sit for 1 minute, then stir until smooth.

Mix in unsalted butter into the warm ganache until fully incorporated; let cool slightly.

Spread ganache on the flat side of half of the cookies and top with the remaining cookies.

Serve at room temperature or chilled, arranged elegantly on a serving platter.