1. Combine the mascarpone, cream, icing sugar and Grand Marnier or vanilla in the bowl of your stand mixer and mix to combine and thicken slightly. Cover with plastic wrap and place in the fridge to firm slightly.
2. Preheat oven to 160°C, try not to use fan bake if you oven allows it. Grease and line the base of a 24 x 30cm Swiss roll pan with baking paper. Use an electric mixer to beat the egg whites just until soft peaks form. Gradually add the caster sugar while continuously whisking. Continue whisking until thick and glossy and sugar dissolves. Add cornflour and vinegar and use a metal spoon to gently fold until just combined. Spread mixture into prepared pan. Bake for 20 minutes or until just firm - it will puff up but don't worry. Remove from oven and set aside for 3 minutes to cool slightly. It will deflate and this is normal. Lay a clean tea towel on a clean work surface. Top with a large sheet of baking paper and dust with extra icing sugar. Turn roulade out onto baking paper and set aside for 10 minutes to cool.
3. Spread mascarpone mixture along the long side of meringue closest to you (having added fruit mince or other flavouring). Carefully roll pavlova, using the paper and tea towel as a guide, to enclose filling. Keep pavlova wrapped in baking paper and tea towel. Transfer pavlova roll to a tray and place in the fridge for 3 hours or overnight to firm up.
Slice into serving slices and serve.