1. Prepare the Cookie Batter:

• Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

• In a bowl, beat the softened butter and sugar together until light and fluffy.

• Mix in vanilla extract and egg whites until smooth.

• Sift in the flour and salt, folding gently until combined. Do not overmix.

2. Shape and Bake:

• Transfer the batter into a piping bag fitted with a small round tip.

• Pipe small, thin squares (about 1.5 inches) onto the prepared baking sheet, leaving space between them.

• Bake for 8-10 minutes, until the edges are lightly golden.

• Let the cookies cool completely on a wire rack.

3. Make the Filling:

• For chocolate ganache: Heat heavy cream until warm, then pour over the melted chocolate. Stir until smooth.

• For buttercream: Beat softened butter and powdered sugar together until fluffy, then mix in vanilla extract.

4. Assemble the Sandwich Cookies:

• Spread or pipe a small amount of filling onto one cookie and gently press another on top.

• Let set at room temperature or refrigerate for a firmer texture.

5. Serve and Enjoy:

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