For the crust in a food processor, combine the flour, polenta, sugar, baking powder, and salt and process just to mix. Add the butter and process in short bursts, scraping down the sides of the bowl as necessary, until the mixture resembles coarse bread crumbs. Add the whole egg, egg yolk, and cinnamon and process again in short bursts, scraping down the sides, until the dough just comes together, but is still a bit crumbly. Add a little of the wine only if the dough seems very dry. Do not overprocess or the crust will be tough. Turn the dough out onto a lightly floured surface, work it together into a ball, press it down into a disk, and wrap with plastic. Refrigerate for at least 1 hour or overnight.

In a medium saucepan combine the plums, cinnamon, sugar, Cognac, and currants. Stir together over low heat until the sugar has dissolved, then cover and simmer for about 15 minutes, or until the plums are very soft. Set the filling aside to cool, uncovered.

Preheat the oven to 350°F. Line a 12-inch tart pan with high sides and a removable bottom with parchment paper.

On a lightly floured surgace roll out the pastry to a 14-inch round, letting it rest for 5 minutes if it is difficult to roll when it first comes out of the refrigerator. Line the tart pan with the dough, pressing it into the corners and up the sides and patching it if necessary. Set the pan on a baking sheet and spoon the filling into the tart shell.

Bake for about 45 minutes or until the filling is bubbly and the crust is golden. Cool on a rack for 15 minutes. Remove the sides of the pan. Serve either slightly warm or at room temperature, with a scoop of lemon ice cream, if desired.