Line a baking sheet with parchment paper; draw 6 (3 1/2") circles onto paper. Turn paper over to prevent markings from touching meringues; set aside.
Beat egg whites at low speed with an electric mixer until foamy; add cream of tartar. Increase speed to medium; gradually add 2 tablespoons sugar, 1 tablespoon at a time, beating until soft peaks form. Increase speed to high; gradually add remaining 6 tablespoons sugar and 1/2 cup powdered sugar, 1 tablespoon at a time, beating until stiff peaks form and sugars dissolve (2 to 4 minutes). Fold in peppermint extract.
Immediately spoon meringue into mounds on paper circles. Shape into circles, using the back of a spoon, and mounding the sides at least 1/2" higher than centers. Bake at 225° for 1 hour and 35 minutes. Turn oven off. Let meringue shells stand in closed oven 8 hours. Peel shells gently from parchment paper. (Store meringue shells in an airtight container up to 2 days before filling.)
Place whipping cream and chocolate morsels in a saucepan. Cook over medium-low heat, stirring constantly, until chocolate melts and mixture is smooth. Remove from heat, and cool completely. Cover and chill at least 8 hours. Beat chocolate mixture at medium-high speed with an electric mixer until thickened and firm.
Spoon mousse evenly into meringue shells; sprinkle with crushed peppermint candies.
Meringue Moxie: Follow the tips below for snow-white, crisp meringues every time.
Want meringues with the most volume?
*Use room temperature eggs. Run eggs (still in the shell) under very warm water 30 seconds; pat dry.
*Beat egg whites in a clean bowl, free of grease; a greasy bowl inhibits fluffy results.
*Use a little cream of tartar; it helps produce a stable meringue that won't deflate. Quickly spoon meringue onto baking sheet, and bake immediately after mixing.
What's the key to crispy?
*Hard meringues have a high sugar content, 1/4 cup per egg white. The sugar draws water away from beaten egg whites so that in the oven the meringues become crisp and dry.
*Using powdered sugar makes hard meringues very light.
Can I make them ahead?
*Hard meringues should keep several days when stored in an airtight container. If needed, recrisp them in a 200° oven for 20 to 30 minutes.