01 - Preheat oven to 225°F and line baking sheet with parchment paper.
02 - Whisk egg whites in clean bowl until soft peaks form. Gradually add caster sugar one tablespoon at time, whisking continuously until stiff glossy peaks form and sugar dissolves completely.
03 - Gently fold cornstarch, vinegar, and vanilla extract into meringue mixture until just combined.
04 - Spoon or pipe small mounds approximately golf ball-sized onto prepared baking sheet, spacing 2 inches apart.
05 - Bake for 1 hour until crisp and dry to touch. Turn off oven, prop door slightly ajar, and let meringues cool completely inside oven.
06 - Combine raspberries, sugar, and lemon juice in small saucepan over medium heat. Cook 5-7 minutes, stirring occasionally, until raspberries break down completely.
07 - Press raspberry mixture through fine mesh sieve into bowl, pressing firmly to extract liquid. Discard seeds. Refrigerate until chilled.
08 - Whisk heavy cream, powdered sugar, and vanilla extract in chilled bowl until soft peaks form.
09 - Carefully cut small cavity in base of each cooled meringue using small knife, removing interior portion to create space for filling.
10 - Fill each hollowed meringue with whipped cream using spoon or piping bag. Drizzle generously with chilled raspberry coulis. Garnish with fresh raspberries and mint leaves if desired. Serve immediately.