Preheat the oven

• Preheat the oven to 175 °C.
• Line two baking sheets with parchment paper.

Cream the butter and sugars

• Add the butter, powdered sugar and granulated sugar to a mixing bowl.
• Cream on medium speed until light and fluffy.

Add the egg and vanilla

• Add the egg and vanilla extract.
• Mix until well incorporated, scraping down the sides as needed.

Make the cookie dough

• Whisk together the flour, cornstarch and salt in a medium bowl.
• Add the dry mixture to the wet mixture and mix just until the batter comes together.

Pipe the cookies

• Transfer the dough to a piping bag fitted with a ½-inch round tip.
• Pipe the dough in smooth, straight lines about 7 cm long, spaced about 4 cm apart.
• Wet the tip of your finger and gently press down on the dough to smooth out any sharp edges.

Bake the cookies

• Bake until the edges are golden brown.
• Remove from the oven and let the cookies cool on the baking sheet.

Make the ganache

• Heat the heavy cream in a saucepan until steaming hot but not boiling.
• Pour it over the chopped chocolate in a heat-safe bowl and let it sit.
• Whisk until the ganache is fully smooth and all of the chocolate is melted.

Whip the ganache

• Beat the ganache with an electric hand mixer on high speed until paler, more airy and slightly thicker.

Fill the cookies

• Transfer the ganache to a piping bag and snip off a small corner.
• Pipe about 2 teaspoons of ganache in the center of the bottom of half of the cookies.
• Place another cookie on top and gently press down so the chocolate spreads to the edges.

Let the chocolate set

• Let the chocolate set, speeding it up in the refrigerator if desired, then enjoy.