For the Pavlova: Preheat the oven to 285 degrees F and line a baking sheet with nonstick baking paper or a silicone mat.
In the clean, dry bowl of an electric mixer fitted with the whip attachment, add the egg whites and whip to soft peaks. With the mixer on medium speed, add the sugar in 3 or 4 stages, mixing well after each addition. Mix until the meringue is thick and smooth, and can stand in firm peaks. Add in the espresso powder, corn flour and vinegar; mix until incorporated.
Spread the Pavlova into an 8.5-inch circle on the prepared baking sheet, leaving the edges slightly higher than the center so that you have a gentle "crater" to fill later.
Bake for 20 minutes, then turn down the heat to 250 degrees F and bake until the outside of the Pavlova is dry to the touch, another 50 to 60 minutes. With the oven off and the door slightly ajar, leave the Pavlova to cool completely. Transfer to a serving plate or an airtight container until needed.
For the spiced plums: Meanwhile, combine the water, honey, orange zest, cinnamon, star anise and vanilla bean in a saucepan; bring to a simmer. Add the plums and simmer gently until soft and easily pierced with a knife, 20 minutes. With a slotted spoon, remove the plums to a bowl and reserve. Reduce the poaching liquid for a further 10 to 15 minutes, until syrupy.
To serve, top the Pavlova with whipped cream, spiced plums, toasted nuts and a drizzle of the reduced poaching liquid. Garnish with mint leaves and serve immediately--otherwise the filling will make the Pavlova soggy.