Add the chopped kiwi fruit to a small saucepan with a couple of tablespoons of sugar. Cook over a medium heat so that the fruit releases its liquid to create a syrup. Bring to a gentle simmer, and cook for another 10 minutes or so, stirring regularly, until the mixture thickens. Be careful, as the jam will be incredibly hot - don't allow the pan to spit. Set aside to cool completely.
Whisk the whipping cream until it forms soft peaks (it's much quicker to do this with an electric mixer!). Add the mascarpone cheese, lemon curd, and most of the cooled kiwi jam (just hold back a tablespoon or so), and mix well to combine.
Smear most of the remaining kiwi jam around the inside of 4 small glasses, and add the kiwi lemon posset. Garnish with any remaining jam, and fresh mint if desired.