Preheat the oven to 300 degrees. Trace a circle around an 8- or 9-inch round cake or pie pan onto a piece of parchment paper. Place the parchment paper marked side down on a baking sheet.
Beat the egg whites and salt until soft peaks start to form. Slowly beat in the sugar, a little at a time. Continue to beat on high speed until stiff peaks form. Beat in the cornstarch, then the vanilla and vinegar. Spread the mixture onto the baking sheet using the circle as a guide. With the back of a spoon, make an indentation in the center of the pavlova. Bake for 60-75 minutes or until crisp around the edges and the center is fully set. Remove from the oven and let cool completely. When cool, remove from the parchment paper and place on a serving plate.
Make the Cranberry Curd:
While the pavlova is baking, make the cranberry curd. Lightly beat the egg yolks with a fork in a bowl and set aside.
Finely grate the zest of the orange and squeeze the juice into a measuring bowl. Add enough water to make a total of ¾ cup. Pour into a medium saucepan, add the zest and cranberries, and cook over medium-high heat, stirring occasionally, until very soft, about 15 minutes.
Place a fine strainer over a bowl. Pour the cranberry mixture into the strainer and press with the back of a spoon, extracting as much juice as possible. Discard the solids. Return the strained cranberries to the saucepan with the sugar and cook over medium heat, stirring, until the sugar has dissolved fully. Whisk about 2 tablespoons of the warm cranberry mixture into the egg yolks, then add the egg yolk mixture to the pan. Continue to cook, stirring constantly, until the mixture thickens, about 4-5 minutes. Remove from the heat and whisk in the butter – whisking until the butter is melted and the mixture is fully combined. Pour the mixture into a bowl, cover with a piece of plastic wrap, pressing the plastic wrap directly onto the surface, and refrigerate until cold, about 1 hour.
Assemble the Pavlovas:
Before serving, put the cranberry curd in the center of the pavlova. Whip the cream with the powdered sugar until soft peaks form and place on top of the cranberry curd. Sprinkle with the chopped pistachios and serve.