1/2 cup (113 grams) unsalted butter, at room temperature
1/2 cup (60 grams) confectioners' sugar
2 cups (260 grams) all-purpose flour
Granulated sugar, for sprinkling
6 cups fresh or frozen blueberries
1/2 cup (100 grams) granulated sugar
2 tablespoons cornstarch
Preheat the oven to 350°F. Grease a 9 x 9-inch baking pan with butter. Line the pan with a piece of parchment paper with 2 sides overhanging the edges of the pan to create handles (this will help easily remove the bars from the pan).
To make the shortbread cookie crust: In a food processor, combine the flour, confectioners’ sugar, butter, thyme, and salt. Process the ingredients until they form a ball of dough. If you’re using very cold butter this may take a couple minutes. (Don't wash the processor bowl, you’ll use it again for the cream cheese dough.)
Press the crust dough into the prepared pan. Lay a piece of plastic wrap over the top and then rub it smooth. Lift the plastic wrap off. Bake the crust on the middle oven rack for about 20 minutes, just until it is golden brown. Allow the crust to cool while you prepare the cream cheese dough and filling.
To make the cream cheese lattice dough: In the food processor, cream together the cream cheese, butter, and confectioners' sugar. Add the flour and pulse the dough until it comes together in a soft ball. Divide the ball into 2 discs, cover with plastic wrap, and refrigerate for about 1 hour.
To assemble the bars, once the cream cheese dough is chilled, roll one of the disks out on a well-floured surface until it’s about 1/8-inch thick. Place it on a baking sheet lined with parchment paper and refrigerate it while you roll out the second disk of dough. Use a pastry wheel to cut the rolled-out dough into 1/2-inch-wide strips.
Next, line another baking sheet with parchment (you’ll create the lattice on this). Lay half the strips over the parchment, each about 1/2 inch apart. Fold up every other strip and lay one of the remaining strips across those still laid flat. Repeat with the next rows until you reach the bottom. Start back at the middle and do the same on the other side. Place the baking sheet with the lattice in the freezer until solid, about 20 minutes.
Meanwhile, prepare the blueberry filling: Place the blueberries in a medium saucepan. In a small bowl, whisk together the sugar and cornstarch, then add it to the blueberries. Cook over low heat until the blueberries thicken, about 5 minutes. When they are done cooking, you will have about 2 1/2 cups of blueberry filling. Allow to cool.
Preheat the oven to 350°F. Pour the cooled blueberry filling over the cooled shortbread crust. Remove the lattice from the freezer, trim excess with a pastry wheel so that it fits in the pan. Carefully lift the lattice onto the filling. If the lattice is chilled well enough this should be easy. If it is still too soft, return it to the freezer until it is harder.
Sprinkle the top of the bars with sugar. Bake 35 to 40 minutes, or until the crust is golden and the filling is bubbling.
Allow the bars to cool completely in the pan. Run a knife around the sides and then use the overhanging parchment paper as handles to remove the bars from the pan and place it on a cutting board. Trim the edges if desired and cut equal square bars. They can be stored at room temperature for a day and then covered and refrigerated or frozen.