Cream the butter and sugar by beating 1 cup of softened butter and 1 cup of powdered sugar on medium speed for 3-5 minutes until light and fluffy.

Incorporate the egg white into the creamed mixture and mix on low speed for 1-2 minutes until smooth.

Enhance flavor by adding 1 teaspoon of vanilla extract, stirring gently until fully incorporated.

Prepare dry ingredients by whisking together 2 cups of flour, 2 tablespoons of cornstarch, and a pinch of salt before gradually adding it to the wet ingredients.

Pipe small oval shapes of dough onto a parchment-lined baking sheet using a piping bag.

Bake in a preheated oven at 325°F (160°C) for 8-12 minutes until edges are lightly golden.

Cool the cookies on the baking sheet for 5 minutes before transferring to a wire rack.

Make ganache filling by chopping 6 ounces of white chocolate, heating ½ cup of heavy cream until simmering, and combining with the chocolate.

Assemble the cookies by spreading ganache on one cookie and topping with another.

Chill the assembled cookies for 30 minutes to firm up the ganache.

Serve the cookies at room temperature or chilled.