Preheat oven to 225°.
To prepare meringues, cover a baking sheet with parchment paper. Draw 6 (4-inch) circles on paper. Turn paper over; secure with masking tape. Place egg whites and cream of tartar in a large bowl; beat with a mixer at high speed until foamy. Gradually add 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form (do not underbeat). Divide egg white mixture evenly among 6 drawn circles on baking sheet; spread to fill the circles using the back of a spoon.
Bake at 225° for 1 1/2 hours. Turn oven off; cool meringues in closed oven for 30 minutes. Carefully remove meringues from paper. Cool completely. (Meringues can be stored in an airtight container for up to a week.)
To prepare chocolate mascarpone, sift together 1 1/2 tablespoons sugar and cocoa. Combine mascarpone, milk, and vanilla in a small bowl; stir just until combined. Stir in cocoa mixture.
To prepare strawberries, toss berries with 1/4 cup sugar; let stand 15 minutes.
Place 1 meringue on each of 6 plates; spread about 1 1/2 tablespoons mascarpone mixture on top of each meringue. Top each serving with about 1/2 cup strawberry mixture. Garnish with mint sprigs, if desired.