Preheat oven to 350°F. Line a 9x13 inch baking pan with parchment paper, leaving a 1-inch overhang on two sides for easy removal. This takes about 2 minutes.

In a medium bowl, whisk together 2 cups all-purpose flour, 1/2 cup granulated sugar, and 1/2 teaspoon salt. Mix for 30 seconds until combined.

Cut 1 cup cold unsalted butter (cubed into 1/4-inch pieces) into the flour mixture using a pastry cutter, two forks, or your fingertips. Work quickly so the butter stays cold. Stop when the mixture looks like coarse breadcrumbs with pea-sized butter pieces visible. This takes 5–7 minutes of rubbing.

Transfer the crust mixture to the prepared pan and press firmly and evenly into the bottom. Use the bottom of a measuring cup to create an even surface. Press down for 2–3 minutes until compact.

Bake the crust at 350°F for 12–15 minutes on the center rack. It should turn light golden brown and smell toasty (do not brown deeply). Checkpoint: The crust is ready when it no longer looks raw and has just started to turn pale gold at the edges.

While the crust bakes, prepare the cream cheese layer. In a large bowl, beat 8 ounces softened cream cheese (cut into small cubes) and 1/2 cup granulated sugar with an electric mixer on medium speed for 2–3 minutes. Scrape down the bowl halfway through. The mixture should look light, fluffy, and pale.

Add 1 large egg to the cream cheese mixture and beat on low speed for 1 minute until just combined. Do not overmix.

Add 1 teaspoon vanilla extract and stir by hand with a spatula for 30 seconds until blended.

Remove the warm crust from the oven. Immediately spread the cream cheese mixture evenly over the hot crust using an offset spatula or the back of a spoon. Work quickly but gently to avoid dragging up bits of crust. The layer should be about 1/4 inch thick.

In a small bowl, combine 2 cups fresh blueberries, 1/4 cup granulated sugar, 2 tablespoons cornstarch, 1 tablespoon fresh lemon juice, 1/4 teaspoon lemon zest, and a pinch of salt. Gently fold everything together with a spoon for 1 minute, being careful not to crush the berries. The blueberries should be coated and the mixture should look slightly thickened.

Scatter the blueberry mixture evenly over the cream cheese layer. Spread gently so the topping covers the filling without pressing down (some spillover onto the crust edges is fine). This takes about 2 minutes.

Bake the entire pan at 350°F for 25–30 minutes on the center rack. The bars are done when the edges of the cream cheese layer look set and slightly puffed, and you see some blueberry juice bubbling gently at the edges (the center may still jiggle slightly, and this is correct). The blueberries should look plump and glossy but still hold their shape.

Remove from the oven and place the pan on a wire cooling rack. Do not move it. Allow to cool at room temperature for 30 minutes (the center will continue to set as it cools).

Transfer the pan to the refrigerator and chill for at least 2 hours (or overnight). Cold bars are much easier to cut and taste better.

Once chilled and firm, lift the parchment paper to remove the entire slab from the pan onto a cutting board. Using a long, sharp knife dipped in warm water (and wiped dry between cuts), slice into 12–16 bars. Wipe the knife between each cut for clean edges.

Serve chilled or at room temperature. Store leftovers covered in the refrigerator for up to 5 days.