INGREDIENTS
For the pastry
2 cups all-purpose flour, plus more for the work surface
1/4 cup confectioners’ sugar, sifted
1/2 teaspoon kosher salt
10 tablespoons unsalted butter, cold, cut into chunks
1 large egg yolk
1/3 cup plus 2 tablespoons whole milk
For the filling
3/4 cup your favorite flavor jam
2 teaspoons cornstarch, mixed with 1 teaspoon cold water
1 large egg, beaten with 1 teaspoon warm water
For the glaze
1 cup confectioners’ sugar, sifted
2 teaspoons whole milk
2 teaspoons corn syrup
1/2 teaspoon vanilla extract
Sprinkles (optional)
DIRECTIONS
Make the dough
1. In a food processor, combine the flour, confectioners’ sugar, and salt and process until blended. Add the butter and process again just until the mixture looks like coarse crumbs. Add the egg yolk and milk and process until the dough just comes together. Dump the dough onto a double layer of overlapping sheets of plastic wrap. Press the dough into a disk, wrap it with the plastic wrap, and refrigerate for at least 30 minutes and up to overnight.
Make the filling
2. In a small saucepan over medium heat, cook the jam and cornstarch, stirring quite frequently but not constantly, until slightly thickened and bubbly. Let cool.
Assemble the toaster tarts
3. Line 2 baking sheets with parchment paper. On a lightly floured work surface, divide the dough in half and form each portion into a rough rectangle. Roll one rectangle until it measures about 16 by 9 inches. Using a ruler and a pizza cutter, cut the rectangle into 12 small rectangles, each about 3 by 4 inches. Set the rectangles on a baking sheet and refrigerate while you repeat with the remaining dough.
4. Lightly brush half the rectangles with the beaten egg. Place a tablespoon dollop of the filling in the center of each egg-brushed rectangle and use the back of the spoon to spread it evenly over the dough, leaving a border of about 1/2 inch. Top each filling-slathered rectangle with a plain rectangle and press the edges together with your fingertips to seal, being careful not to let the filling ooze out the sides. Crimp the edges with the tines of a fork. Place 6 tarts on each baking sheet, spacing them evenly, Prick the tarts all over with the fork. Refrigerate while the oven preheats.
5. Preheat the oven to 375°F (190°C) and position 2 oven racks in the upper and lower third of the oven. Bake the tarts, rotating the pans once halfway through, until golden brown, 15 to 18 minutes. Let cool on a wire rack.
Make the glaze
6. Meanwhile, whisk together the confectioners’ sugar, milk, corn syrup, and vanilla until smooth. Set aside at room temperature until needed.
7. Stir the glaze to recombine. Drizzle or smear the glaze on the cooled tarts. If desired, decorate with sprinkles.