Pulse the flour, sugar, salt and zest 2 or 3 times in the bowl of a food processor fitted with a steel blade. Add the butter and pulse until the mixture resembles coarse crumbs.
In a separate bowl, beat the egg yolks together with the cold vin santo. With the machine running, add the egg/sherry mixture to the dough through the feed tube. Process 5 to 6 seconds or until the mass combines and leaves the sides of the bowl. Permit the pastry to rest fir 30 minutes rest, wrapped securely in plastic, in a cool place.
Thin the cherry jam a bit with a little hot water, stirring well to combine. Add a few grindings of pepper to heighten the flavor of the cherries and stir again. Set aside.
Preheat the oven to 425 degrees F. Roll out the dough in a free-form circle of about 12 inches, and transfer it to a buttered baking sheet or a sheet lined with parchment. Leaving a 2-inch border uncovered, spread the cherry mixture over the dough, folding, pleating, and tucking the border of the dough up over the fruit.
Bake the tart for 20 to 25 minutes, or until the pastry is deeply golden and the fruit is glossy. Cool on a rack for several minutes and serve warm with a glass of the vin santo.