How to Make It
Preheat the oven to 350°. In a bowl, using an electric mixer, beat the butter and sugar at medium speed until fluffy, 2 minutes. Beat in the egg yolks and vanilla. Beat in the flour, cornmeal and salt and until a dough forms. Gather the dough into a ball and transfer to a large nonstick baking sheet. Cover with plastic wrap and roll it out to a 9-inch square, a scant 1/4 inch thick. Bake for about 10 minutes, until the edges are lightly browned. Immediately transfer the dough to a food processor and pulse until fine. Lower the oven temperature to 300°.
Line a 13 3/4-by-4-inch, straight-sided rectangular tart pan with a removable bottom with foil. Spray the foil with vegetable spray. Press a 1/2-inch layer of the crumbs evenly over the bottom and up the side of the pan. Refrigerate until firm, 20 minutes.
In a medium bowl, using an electric mixer, beat the butter with the sugar at medium speed until fluffy, about 2 minutes. Add half of the mascarpone cheese and beat at low speed until combined. Add the remaining mascarpone cheese, the egg, vanilla, lemon zest and cream and beat just until smooth, about 1 minute. Pour the filling into the crust and smooth the surface.
Set the tart pan on a sturdy baking sheet. Position racks in the bottom and center of the oven. On the bottom rack, place a roasting pan filled with 1 inch of water. Place the tart on the center rack and bake for 35 to 40 minutes, until the filling is puffed and set. Let cool to room temperature, then refrigerate until chilled, at least 2 hours. Lift the tart from the pan and remove the foil. Transfer to a platter.
In a small saucepan, bring the sugar, Muscat and lemon juice to a boil. Reduce the heat to moderate and simmer until syrupy, about 10 minutes. Let cool slightly, then transfer to a bowl and add the cherries. Let stand for 10 minutes. Using a slotted spoon, transfer the cherries to the tart and serve.