Preheat oven to 200ºF. Line two baking sheets with parchment paper or foil.
Put the egg whites and cream of tartar in a clean and dry large bowl. Using an electric mixer, beat on medium speed until the whites form soft peaks when the beaters are lifted, 3 to 5 minutes. Gradually add sugar, beating until the whites resemble shaving cream and hold stiff peaks when the beaters are lifted, 3 to 5 more minutes. Fold in the crushed candy and vanilla extract.
Spoon meringue into a large gallon-size ziplock bag, cut off a small corner, and squeeze the meringue into 1 1/2-inch rounds on the 2 baking sheets.
Bake without opening oven door for 1 hour and 15 minutes. Turn off the oven and let the meringues sit in the oven until cool, at least another hour or overnight. Serve immediately or store in an airtight container for about a day. The meringues will keep longest if the humidity is low.
Tip: Meringue cookies get soft and sticky in humid conditions. For the most crunchiness, make the cookies when the humidity is low.