Make the Cookies

In the bowl of a stand mixer fitted with a paddle attachment, cream the butter, sugar, and brown sugar on medium-high for 2 minutes.

Add the salt and vanilla, and beat for 30 seconds.

Add the flour, and beat for 30 seconds. The dough will be very stiff.

Shape the dough into a ball, place it in a bowl, and cover with plastic wrap. Chill for at least one hour, or overnight.

When ready to bake, preheat the oven to 350°F. Unwrap the dough, and shape into golf-ball sized balls, using about 2 tablespoons of dough for each.

Place the dough balls on a parchment-lined cookie sheet, and press down the centers to create a well for the chocolate filling. Each cookie should be shaped like a disc with a lip around the outer edge. The cookies should be about 3 inches in diameter, and set about 2 inches apart on the cookie sheet.

Bake the cookies for 9 minutes. They will be soft, and the edges may be slightly golden.

Remove the cookies from the cookie sheet and transfer to a rack to cool completely.

Make the Ganache

Heat the heavy cream in a microwave-safe bowl in the microwave for 1 minute. 

Combine the remaining ganache ingredients in a separate bowl. Pour the hot cream over the ganache ingredients, and stir to melt. Keep stirring until completely smooth. 

Set the ganache aside to cool completely, about 15 minutes.

Spoon the cooled ganache into a piping bag, and cut the tip to make a nickel-sized hole. Pipe a thick line of chocolate on the cookies on one side, just inside the lip. The chocolate ganache should fill the cookies just about halfway.

Let the chocolate ganache set while you make the meringue.

Make the Meringue

Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment. Whip the egg whites on medium speed for 1 minute, or until a slight microfoam has begun to form. 

Add in the salt and cream of tartar, and continue whisking for another minute until a foam begins to form with large bubbles. With the mixer still running, slowly stream the sugar into the egg whites. Continue whisking for 3 minutes, or until very soft peaks have formed.

Add in the cornstarch and continue to whisk on medium speed, for 6 to 8 minutes or until the meringue has thickened and stiffer peaks have formed. Lastly, add in the vanilla and whisk for another 2 minutes, or until stiff peaks are visible. Spoon the meringue into a piping bag fitted with a star piping tip.

Pipe the meringue onto the cookies, opposite the chocolate. Pipe in a line, swirl, or curve. Use a kitchen torch or camping torch to toast the meringue. Be sure to keep the torch about 2 - 3 inches away from the meringue to avoid burning it.