How to Make It
In a large bowl, whisk the flour with the sugar and salt. Using your fingertips, blend in the butter until pea-size pieces form. Add the buttermilk and vinegar and mix with a wooden spoon until a dough forms. Pat into a 1-inch-thick disk, wrap in plastic and refrigerate for 30 minutes.
On a sheet of lightly floured parchment paper, using a lightly floured rolling pin, roll out the pastry dough to a 14-inch round that's 1/8 inch thick. Transfer to a baking sheet and freeze for 15 minutes.
In a large bowl, combine the plums with the 3 tablespoons of sugar, the cornstarch and lemon zest and toss to coat.
Preheat the oven to 425°. Spread the cherry jam in an 8-inch square in the center of the dough round, leaving about 3 inches around. Scatter the Luxardo cherries, if using, on top. Layer the plums, slightly overlapping, over the jam. Fold the rim of dough over the filling, leaving the center exposed and tucking in the corners to create a square crostata. Brush the dough with cream and sprinkle with sugar. Bake for 15 minutes, then lower the oven temperature to 375° and bake for about 30 minutes longer, until the pastry is deep golden and the filling is bubbling and slightly runny. Transfer to a rack to let cool until warm. Serve with whipped cream or ice cream.
Unlike cloyingly sweet, electric-red maraschinos, Luxardo cherries are dark brown and tart, and they have a deep cherry flavor. They're available at specialty food shops and from amazon.com.