Preheat your oven to 350°F (175°C) and prepare a baking sheet lined with parchment paper.

In a mixing bowl, combine softened unsalted butter and powdered sugar, and beat on medium speed for about 3-5 minutes until light and fluffy.

Add a large egg and pure vanilla extract to the butter-sugar mixture, mixing on low speed for 1-2 minutes until smooth.

Sift together all-purpose flour and baking powder, then gradually add this dry mixture to the wet ingredients, mixing gently to combine.

Shape the cookie dough into discs, about 1 inch thick, and place them on the prepared baking sheet.

Bake the cookies in the preheated oven for 15-20 minutes until the edges are lightly golden.

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

Melt your white chocolate in a microwave-safe bowl in short intervals of 20-30 seconds, stirring in between until smooth.

Spread a layer of melted white chocolate onto the flat side of one cookie and place another cookie on top, pressing gently to create a sandwich.

Let the assembled treats set at room temperature for about 30 minutes to allow the chocolate to firm up.