01 - Preheat the oven to 230°F and line a baking tray with parchment paper.
02 - In a clean bowl, whisk the egg whites until soft peaks form. Gradually add caster sugar, one tablespoon at a time, whisking until stiff, glossy peaks form and sugar is completely dissolved.
03 - Gently fold in cornstarch, vinegar, and vanilla extract until just combined.
04 - Spoon or pipe small mounds, approximately golf ball-sized, onto the prepared baking tray, spacing them well apart.
05 - Bake for 1 hour until crisp outside. Turn off oven and let meringues cool inside with door slightly ajar.
06 - Combine raspberries, sugar, and lemon juice in a small saucepan. Cook over medium heat for 5 to 7 minutes, stirring constantly, until raspberries break down completely. Press through a fine sieve to remove seeds. Refrigerate until chilled.
07 - Whisk heavy cream, powdered sugar, and vanilla extract until soft peaks form.
08 - Once meringues are completely cool, carefully cut a small cavity in the base of each. Fill with whipped cream using a spoon or piping bag.
09 - Drizzle generously with chilled raspberry coulis. Garnish with fresh raspberries and mint leaves if desired. Serve immediately for optimal texture.