Step 1: Preheat the Oven and Prepare the Tray

Preheat your oven to 160°C (140°C fan) or 320°F. Line a large baking tray with parchment paper, ensuring it hangs over the edges for easy removal.

Step 2: Make the Meringue

In a clean, dry mixing bowl, whisk the egg whites on medium speed until soft peaks form. Gradually add the caster sugar, a spoonful at a time, while continuing to whisk until the meringue is thick and glossy. This should take about 5 minutes. Add the vinegar and cornstarch, then gently fold them into the meringue.

Step 3: Spread and Bake the Meringue

Spoon the meringue onto the prepared baking tray, spreading it evenly to the edges. Smooth the top with a spatula. Bake in the preheated oven for 25-30 minutes, or until the meringue is just set and lightly golden on top. The meringue should have a crisp exterior and a marshmallow-like interior.

Step 4: Cool the Meringue

Remove the meringue from the oven and let it cool for 5 minutes. Place a large piece of parchment paper or a clean tea towel on a flat surface, dust lightly with icing sugar to prevent sticking. Invert the meringue onto the dusted surface and carefully peel away the baking parchment. Let it cool completely.

Step 5: Prepare the Filling

In a mixing bowl, whip the double cream with icing sugar and vanilla extract until it forms soft peaks. Be careful not to overwhip.

Step 6: Assemble the Roulade

Spread the whipped cream evenly over the cooled meringue, leaving a small border around the edges. Scatter the cranberries or raspberries over the cream.

Step 7: Roll the Roulade

Using the tea towel or parchment paper to help, gently roll up the meringue from one short end, keeping it as tight as possible without pressing too hard. The meringue may crack slightly, which is normal and adds a rustic look.

Step 8: Chill and Serve

Transfer the roulade seam-side down onto a serving platter. Chill in the refrigerator for at least 30 minutes to set. Dust with icing sugar and garnish with fresh mint sprigs before serving.