01 - Preheat oven to 230°F and line a baking sheet with parchment paper.
02 - In a clean bowl, beat egg whites with electric mixer until soft peaks form. Gradually add caster sugar, 1 tablespoon at a time, beating continuously until stiff, glossy peaks form and sugar is completely dissolved.
03 - Gently fold cornstarch, vinegar, and vanilla extract into the meringue mixture using a spatula until just combined.
04 - Spoon or pipe meringue into small mounds, about golf ball-sized, onto prepared baking sheet, spacing 2 inches apart.
05 - Bake for 1 hour until exterior is crisp and dry. Turn off oven and leave meringues inside with door slightly ajar to cool completely, about 1 additional hour.
06 - Combine raspberries, sugar, and lemon juice in small saucepan over medium heat. Cook 5 to 7 minutes, stirring occasionally, until raspberries break down completely. Press mixture through fine-mesh sieve into bowl to remove seeds. Refrigerate until chilled.
07 - Beat heavy cream, powdered sugar, and vanilla extract in chilled bowl until soft peaks form. Do not overbeat.
08 - Using small knife, carefully carve out shallow cavity in bottom of each cooled meringue. Spoon or pipe whipped cream filling into cavities.
09 - Drizzle filled meringues generously with chilled raspberry coulis. Garnish with fresh raspberries and mint leaves if desired. Serve immediately for optimal texture contrast.