Step 1: Making the Cookie Dough

1. Preheat the oven to 300°F (150°C). Line a baking sheet with parchment paper.

2. In a bowl, beat the softened butter and powdered sugar until smooth and creamy.

3. Mix in the vanilla extract and egg white, beating until fully incorporated.

4. Gently fold in the sifted cake flour, mixing until just combined.

Step 2: Shaping and Baking the Cookies

5. Transfer the dough into a piping bag or use a small spoon to create thin, 1.5-inch (4 cm) squares or circles on the prepared baking sheet. Leave space between each cookie as they will spread.

6. Bake for 10-12 minutes, or until the edges turn a light golden brown.

7. Remove from the oven and let the cookies cool completely on a wire rack.

Step 3: Preparing the White Chocolate Filling

8. In a heatproof bowl, melt the white chocolate and heavy cream together over a double boiler or in the microwave in 10-second intervals, stirring until smooth.

9. Let the chocolate cool slightly until it reaches a spreadable consistency.

Step 4: Assembling the Cookies

10. Spread a thin layer of melted white chocolate filling onto the flat side of a cookie.

11. Place another cookie on top, gently pressing to create a sandwich.

12. Repeat with the remaining cookies.

Step 5: Serving and Storing

13. Let the cookies rest for at least 30 minutes to allow the chocolate to set.

14. Store in an airtight container at room temperature for up to 3 days or refrigerate for up to a week.