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 on medium speed until soft peaks form. Increase speed to high and gradually add caster sugar, one tablespoon at a time, continuing to beat until stiff, glossy peaks form and sugar is completely dissolved, about 5-7 minutes.
03 - Gently fold cornstarch, vinegar, and vanilla extract into the meringue using a silicone spatula, being careful not to deflate the mixture.
04 - Spoon or pipe meringue into golf ball-sized mounds onto prepared baking sheet, spacing 2 inches apart. Use the back of a spoon to create a slight indentation in the center of each mound if desired.
05 - Bake for 1 hour until crisp and dry to the touch. Turn off oven and leave meringues inside with door slightly ajar to cool completely, about 1-2 hours. This prevents cracking.
06 - While meringues bake, combine raspberries, sugar, and lemon juice in a small saucepan over medium heat. Cook for 5-7 minutes, stirring occasionally, until raspberries break down and release their juices. Press mixture through a fine-mesh sieve into a bowl to remove seeds. Refrigerate until ready to serve.
07 - In a chilled bowl, whip heavy cream, powdered sugar, and vanilla extract with electric mixer on medium-high speed until soft peaks form, about 2-3 minutes. Do not overwhip.
08 - Using a small, sharp knife, carefully cut a shallow cavity in the bottom of each cooled meringue. Spoon or pipe whipped cream into each cavity. Place filled meringues cream-side up on serving plates.
09 - Drizzle generously with chilled raspberry coulis, allowing it to cascade down the sides. Garnish with fresh raspberries and mint leaves if using. Serve immediately for optimal texture contrast.