Preheat oven to 150ᵒC. Line an oven tray with baking paper, set aside. To separate the eggs, crack an egg over a bowl and tip the yolk back and forth between shell halves, allowing egg white to drip into the bowl. Take care to ensure no yolk gets into the whites because it prevents proper aeration. Put the egg whites and ¼ tsp white vinegar in a, clean, dry bowl and beat with an electric mixer until soft peaks form.
Slowly add sugar to egg white mixture, gradually beating between each addition. Beat for 5-10 minutes or until sugar is dissolved and mixture is smooth, stiff and glossy.
Add sifted cocoa and cornflour to the mixture add remaining vinegar and vanilla and 6 chopped Tim Tam biscuits, then use a metal spoon to fold in until just combined. Spoon mixture onto prepared tray.
Smooth pavlova mixture using a spatula into a rectangular shape around 5cm deep then smooth top again. Bake for 20 minutes, then reduce oven temperature to 120ᵒC. bake for a further 1 hour and 40 minutes or until shell is pale and crisp. Turn off oven and cool pavlova in an oven with the door ajar.
To serve, put the pavlova on a plate. Put the thickened cream in a bowl and whisk until soft peaks form. Spoon cream on top of pavlova, garnish with remaining chopped Tim Tam, and berries.