This giant grape toaster pastry is ideal for breakfast but is also wonderful served warm as a dessert for a crowd and topped with vanilla ice cream. To warm up individual pieces, use a toaster oven — not an upright toaster or else the filling and glaze will run — to gently warm the pastry before serving. I use heavy cream to brush the outside of the tart so as not to waste an egg for egg wash; use whichever you prefer. I also find that the amount of cornstarch called for gives the filling the perfect texture that’s soft but cuts cleanly. If you want a thinner filling, go with 2 tablespoons; for a firmer hold, use 3 tablespoons. This tart also works well as a template for other berries, like blueberry, blackberry and raspberry.
Wash and dry the Thomcord grapes then pour them into a medium saucepan. (If using Concord grapes, see note below.) Using a potato masher, break open all the grapes to give off their juices. Place the pan over medium heat and bring to a simmer. Reduce the heat to maintain a gentle simmer, cover with the lid slightly ajar and cook until the grapes are very soft, 6 to 8 minutes. Pour the cooked grapes into a bowl then rinse and dry the pan.
In the pan, whisk together the granulated sugar, cornstarch and fine salt. Pour the cooked grapes into the pan (reserve the bowl) and stir to combine with the sugar mixture. Place over medium heat, stirring to dissolve the sugar, then cook, stirring often, until the mixture bubbles all over and thickens. Remove the pan from the heat, stir in the lemon juice and vanilla and scrape the filling into the reserved bowl. Let the filling cool completely.
Working on a floured surface, roll one dough disk into a large rectangle, about 1/8-inch thick. Using a ruler, trim off the edges to give you a 10- by 15-inch rectangle. If there are cracks in the trimmed rectangle, use the scraps to patch them by pressing them onto the rectangle and re-trimming any excess pastry. Slide this rectangle onto a sheet of parchment paper then slide the paper onto a baking sheet. Scrape the cooled grape filling onto the pastry and spread it evenly over the dough, leaving a 1-inch gap on all sides.
Roll and trim the second dough disk in the same manner as the first. Using a pastry brush, brush the exposed dough border of the first rectangle with water to moisten it. Roll the second dough rectangle onto your rolling pin, then unroll it over the filling, lining up the edges of the pastry rectangles with each other. Press along the border lightly to adhere the pastry together (it’s OK if some of the filling comes through) then use a fork to crimp the edges shut all the way around. Using the tip of a paring knife, cut about 12 to 14 small slits across the top pastry to allow the filling to vent off steam. Brush the whole tart with the heavy cream then sprinkle with a pinch of flaky sea salt. Transfer the baking sheet to the freezer for at least 30 minutes or up to 2 days.
Heat the oven to 400 degrees. Transfer the tart from the freezer directly into the oven and bake for 30 minutes. Reduce the oven temperature to 375 degrees and continue baking until the pastry is golden brown all over and the filling is bubbling in the center, 25 to 35 minutes more. Remove the sheet from the oven and let the tart cool completely.
Meanwhile, make the glaze: Place the powdered sugar in a small bowl. Add half the milk and stir to form a thick paste. Add the rest of the milk, 1 teaspoon at a time, until the paste thins to the proper consistency to drizzle but stay put. Stir in the food colorings, if you like.
Once the tart is fully cooled, drizzle the purple glaze over the top then sprinkle with some decorating sugars while the glaze is still wet. Let the tart stand for another 10 minutes to set the glaze before serving.
Basic Pie Pastry Dough
In a large bowl, whisk together the flour, sugar and salt. Add the butter and use your fingers to pinch and rub it into the flour repeatedly until pea-size crumbles form. Pour in the water and use a fork to toss (not stir) the butter-flour mixture until it’s evenly moistened.
Scrape the crumbly mass onto a clean work surface and pat and smoosh gently until it starts to form a solid piece of dough. Cut the dough in half then flatten each piece into a disk, patting the edges to smooth them. Wrap each disk in plastic wrap and refrigerate for at least 1 hour or up to 2 days.
After 2 days, freeze the pastry to store long-term, letting it thaw for one day in the refrigerator before using.
Finely chop the skins by hand (or use a food processor) and stir them into the strained pulp. Pour this mixture into the pan with the sugar, cornstarch and salt and proceed as directed in Step 2 to cook the filling.
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