For the dough: To the bowl of a food processor, add the butter, flour, granulated sugar, salt and lemon zest. Pulse until it resembles coarse sand. Add the egg yolk and a tablespoon of water. Pulse until the mixture forms a crumbly dough, but holds its shape when pinched. (If too crumbly, add a little bit more water.)
Turn the dough out onto a clean work surface. Knead very lightly until the dough comes together. Pat into a disc, then roll into a 1/4-inch-thick circle, turning the dough regularly and flouring as needed to ensure it doesn't stick.
Spray six 4-inch ring molds with the nonstick spray. Cut out 4 dough rounds slightly larger than 4 inches, then transfer a piece of dough to the center of each ring mold, bringing it up the edges to make a rustic tart crust. Slide a spatula underneath to transfer to a half-sheet tray lined with parchment paper. Place in the fridge to chill for 30 minutes.
For the filling: Preheat the oven to 350 degrees F.
Brown the butter in a medium skillet over medium-high heat, cooking until it is light brown in color and smells nutty. Transfer the browned butter to the bowl of a stand mixer fitted with a paddle attachment. Add the granulated sugar and vanilla extract. Turn on the mixer and gradually add the flour. Add 3 eggs, one at a time. Scrape down the sides and mix to combine.
Remove the dough from the fridge. Fill the tarts evenly with the filling and top with the grapes. Make an egg wash by beating the remaining egg in a bowl with a bit of water and brush the crusts. Sprinkle with the turbinado sugar. Transfer to the oven and bake for 20 minutes.
To finish: Add the vin santo and lemon juice to a medium saute pan. Turn on the heat to medium-low and reduce until thick and caramel-like, 5 to 6 minutes. Carefully add the heavy cream and swirl the pan to incorporate. Allow the mixture to come back to a boil, then lower the heat add the butter one pat at a time.
Spoon the mascarpone into quenelles (little oblong dollops) and place on a sheet tray lined with parchment. Place in the fridge to chill for 10 to 15 minutes.
To plate: Spoon a bit of vin santo caramel on the bottom of a plate and dot with the vin cotto. Place the crostata down and top with a mascarpone quenelle. Garnish with thyme leaves and powdered sugar. Repeat to make 5 more servings.