Step 1: Preheat your oven to 250°F (120°C) and line a large baking sheet with parchment paper, drawing an 8-inch circle as a guide.
Step 2: In a clean, grease-free bowl, beat the egg whites until soft peaks form. Gradually add the caster sugar one tablespoon at a time, beating on high until the mixture is thick, glossy, and the sugar has dissolved.
Step 3: Gently fold in the white vinegar, cornstarch, and vanilla extract using a spatula, being careful not to deflate the meringue.
Step 4: Pile the meringue onto the parchment paper within the circle guide. Use a spatula to smooth the sides and create a slight well in the center.
Step 5: Reduce the oven temperature to 200°F (95°C) and bake for 90 minutes until the outside is firm and dry.
Step 6: Turn off the oven and leave the pavlova inside with the door slightly ajar until completely cool (at least 2-3 hours).
Step 7: Prepare the blackberry coulis by simmering half of the berries with lemon juice and a tablespoon of sugar for 5 minutes, then straining and cooling.
Step 8: Whip the heavy cream with powdered sugar until soft peaks form. Top the cooled meringue with cream, remaining fresh berries, and the blackberry coulis.