Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang on the long sides for easy lifting. Lightly grease the paper.

Make the crust and crumble. In a large bowl, whisk the melted butter, granulated sugar, brown sugar, and vanilla. In another bowl, whisk flour, baking powder, and salt. Stir dry into wet until it looks like damp sand and starts to clump. Press about 2/3 of the mixture firmly into the bottom of the pan. Set the rest aside for the topping.

Par-bake the crust. Bake the base for 10–12 minutes until it looks set and just barely golden at the edges. This keeps it crisp under the creamy filling.

Mix the cream cheese filling. Beat the cream cheese, sugar, egg, vanilla, lemon juice, zest, and a pinch of salt until completely smooth. Scrape the bowl so there are no lumps. The texture should be thick but pourable.

Toss the blueberries. In a small bowl, gently mix blueberries with sugar, cornstarch, and lemon juice until coated. The cornstarch thickens the juices as they bake.

Assemble. Pour the cream cheese mixture over the warm crust and spread evenly. Scatter the blueberries across the top. Crumble the remaining dough over the berries, leaving some fruit peeking through.

Bake. Return the pan to the oven and bake 30–35 minutes, or until the top is lightly golden and the center looks set but still has a slight jiggle. Don’t overbake—the bars firm up as they cool.

Cool and chill. Let the pan cool on a rack to room temperature, then chill at least 2 hours (or overnight) before slicing. This helps the layers set cleanly.

Slice and serve. Use the parchment overhang to lift the slab out. Dust with powdered sugar or add a little extra zest if you like. Cut into squares or rectangles.