01 - Preheat oven to 225°F and line a baking sheet with parchment paper.
02 - In a clean, dry bowl, beat egg whites with electric mixer until soft peaks form. Gradually add caster sugar one 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 meringue mixture until just combined.
04 - Spoon or pipe meringue into golf ball-sized mounds 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 until completely cool.
06 - Combine raspberries, sugar, and lemon juice in small saucepan over medium heat. Cook 5-7 minutes, stirring frequently, until raspberries break down completely. Press mixture through fine-mesh sieve to remove seeds. Refrigerate until chilled.
07 - Beat chilled heavy cream, powdered sugar, and vanilla extract in cold bowl until soft peaks form.
08 - Using small knife, carefully carve cavity in base of each cooled meringue. Fill cavities with whipped cream using piping bag or small spoon.
09 - Drizzle raspberry coulis generously over filled meringues. Garnish with fresh raspberries and mint leaves if desired. Serve immediately for optimal texture contrast.