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 continuously until stiff, glossy peaks form and sugar is completely dissolved.
03 - Gently fold in cornstarch, white 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 to allow for even air circulation.
05 - Bake for 1 hour until the exterior is crisp. Turn off the oven and let the meringues cool inside with the door slightly ajar to prevent cracking.
06 - Combine raspberries, sugar, and lemon juice in a small saucepan. Cook over medium heat for 5 to 7 minutes, stirring frequently, until raspberries completely break down. Press through a fine sieve to remove seeds. Refrigerate until chilled.
07 - Whisk heavy cream, powdered sugar, and vanilla extract together until soft peaks form.
08 - Once meringues are completely cool, carefully cut a small cavity in the base of each. Fill each cavity with the prepared whipped cream.
09 - Drizzle generously with chilled raspberry coulis. Garnish with fresh raspberries and mint leaves if desired. Serve immediately to maintain optimal crisp texture.