Preheat the oven to 140°C/120°C Fan/Gas 1. Line a large baking sheet with non-stick baking parchment and draw a 23cm diameter circle on it.
Whisk the egg whites in a clean dry bowl until they form stiff peaks. Gradually beat in the sugar a little at a time, until the meringue is stiff and glossy. Whisk in the vinegar, cornflour and vanilla extract.
Spoon the meringue on to the circle you have drawn on the baking parchment, swirling it out to the edges and making a slight indent in the middle.
Cook the meringue in the preheated oven for 75 to 90 minutes, then turn off the oven, leaving the meringue inside until it is completely cold.
Gently remove the baking parchment from the meringue base and place the base on a serving plate. Whip the cream and icing sugar until it stands in stiff peaks, then spread it over the meringue.
Break the chocolate into squares and melt it in a bowl in the microwave in 30-second bursts, taking out the bowl and stirring the chocolate between bursts. Alternatively, melt it in a bowl set over a pan of gently simmering water.
Drizzle the melted chocolate in a zigzag pattern over the top of the pavlova. Decorate the pavlova with the Cadbury Mini Eggs and cut it into slices to serve.