01 - Preheat oven to 230°F and line a baking sheet with parchment paper.
02 - Whisk egg whites in a clean bowl 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 cornstarch, vinegar, and vanilla extract into the meringue mixture until just combined.
04 - Spoon or pipe small mounds approximately golf ball-sized onto the prepared baking sheet, spacing well apart.
05 - Bake for 1 hour until crisp and dry on the exterior. Turn off oven and leave meringues inside with door slightly ajar until completely cool.
06 - Combine raspberries, sugar, and lemon juice in a small saucepan. Cook over medium heat for 5 to 7 minutes, stirring occasionally, until raspberries break down completely. Press through a fine-mesh 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 cavities with prepared whipped cream.
09 - Drizzle generously with chilled raspberry coulis. Garnish with fresh raspberries and mint leaves if desired. Serve immediately for optimal texture.