Heat the oven to 350°F. Line the bottom and sides of an 8x8-inch square metal baking pan (about 2 inches tall) with 2 sheets of parchment paper, positioning them perpendicular to each other and making sure they are long enough to hang over all four sides by at least 1 inch to form a sling.

Break 16 whole graham cracker sheets into rough pieces and place in a food processor. Add ¼ cup granulated sugar and ¼ teaspoon of the kosher salt. Process until the crackers are the texture of sand, about 15 seconds.

Add 9 tablespoons melted unsalted butter and pulse to combine, 8 to 10 (1-second) pulses. (If using graham cracker crumbs, just stir the crumbs and butter together in a medium bowl.) Reserve ⅔ cup of the mixture for topping.

Transfer the remaining mixture to the baking pan. Use the bottom of a flat measuring cup or your fingers to press the crumbs into an even layer, making sure to reach the edges and fill the corners.

Bake until fragrant and starting to darken around the edges, 12 to 15 minutes. Let cool slightly while you prepare the filling.

Wipe the food processor clean with paper towels. Place 16 ounces room-temperature cream cheese in the food processor. Process until very smooth and fluffy, 1 to 2 minutes. Scrape down the sides of the bowl.

Add ½ cup granulated sugar, 2 teaspoons finely grated lemon zest, 3 tablespoons lemon juice, 1 teaspoon vanilla extract, and ¼ teaspoon kosher salt. Process until combined, about 30 seconds. Scrape down the sides of the bowl. With the motor running, add 2 room-temperature large eggs one at a time and process until just incorporated.

Remove the blade. Add 1 ½ cups of the blueberries into the food processor bowl and gently stir to combine. Transfer to the graham cracker crust and spread into an even layer. Scatter the remaining ½ cup blueberries over the top. Sprinkle with the reserved crumb mixture.

Bake until the edges are set and puffed and the center is still slightly jiggly, 30 to 35 minutes. Place the pan on a wire rack and let cool completely, about 2 hours. Cover and refrigerate until chilled, at least 3 hours or up to overnight.

Grasping the excess parchment, lift the slab out of the pan and place on a cutting board. Dust with powdered sugar if using. Cut into 9 or 12 bars.