Method:

1. Preheat the oven to 175 C¨. Line two baking sheets with parchment paper.

2. Make the cookies: In the bowl of an electric mixer fitted with the paddle attachment, cream the butter with the confectioners' sugar until light and fluffy, 4 to 5 minutes. Add the vanilla extract and mix to combine, about 1 minute.

3. Add the egg whites gradually, mixing until fully incorporated, 2 to 3 minutes. Add the flour and salt and mix just until incorporated, 1 to 2 minutes.

4. Transfer the batter to a ziplock plastic bag. Cut a ¼-inch opening at one of the bottom corners. Pipe the batter into lines 1½ inches long on the baking sheet, leaving at least 2 inches between each cookie. (Tip: Draw lines with a straight edge on the parchment paper, then turn the paper upside down and use the lines as a guide.)

5. Bake until the cookies are set and the edges are pale golden, 9 to 11 minutes. Remove from the oven and cool completely.

6. MAKE THE GANACHE: While the cookies cool, place the chocolate in a medium-size heat-safe bowl. In a small pot, bring the cream to a boil over medium heat. Pour the hot cream over the chocolate. Let it sit for 30 to 45 seconds, then stir to combine.

7. ASSEMBLE THE COOKIES: When the cookies are cool, turn half of them over and drop 1 tablespoon of the ganache onto the center of each upside-down cookie. Top with another cookie and press gently so that the ganache runs to the edges.