Combine the blueberries, 50 g sugar, and 15 ml lemon juice in a small heavy-bottomed saucepan. Cook over medium heat for 4 to 6 minutes, stirring and crushing about one-third of the berries, until the juices simmer. Stir the cornstarch with 15 ml cool water, pour it into the berries, and boil gently for 1 full minute while stirring. The topping should look glossy and leave a brief trail across the pan. Transfer it to a shallow bowl and cool completely for about 45 minutes.

Line a 20 cm square metal pan with two crossing strips of parchment, leaving enough overhang to lift out the chilled cheesecake. Mix the finely ground graham cracker crumbs and 25 g sugar in a medium bowl, then stir in the melted butter until every crumb is evenly moistened and resembles damp sand. Tip the mixture into the pan and compress it firmly into an even layer using the flat bottom of a measuring cup. Refrigerate the crust for at least 20 minutes while preparing the filling.

Pour 45 ml cool water into a small microwave-safe bowl, sprinkle the powdered gelatin evenly over the surface, and leave it undisturbed for 5 minutes. The granules should absorb the water and form a thick, wrinkled gel with no dry patches. Heat it in 5-second microwave bursts, stirring after each, just until completely fluid and clear. Do not let it boil. Set the melted gelatin aside briefly, but use it while still liquid so it does not form strands in the filling.

Place the softened cream cheese, 150 g sugar, lemon zest, 45 ml lemon juice, vanilla, and salt in a large bowl. Beat with an electric mixer on medium-low speed for 2 to 3 minutes, scraping the bowl and beaters twice, until smooth, glossy, and free of visible cream cheese lumps. Blend about 60 ml of this mixture into the liquid gelatin to equalize their textures, then pour it back into the main bowl. Mix on low for 20 seconds, stopping as soon as the gelatin is evenly incorporated.

In a clean, cool bowl, whip the cold heavy cream on medium speed for 2 to 3 minutes, or until soft peaks curl over when the whisk is lifted. Avoid stiff peaks because overwhipped cream will not fold smoothly into the dense cheese base. Fold one-third of the whipped cream into the lemon mixture with a silicone spatula to lighten it. Add the remainder in two portions, sweeping down the bowl and turning the mixture over until no white streaks remain and the filling holds soft, rounded folds.

Scrape the lemon cheesecake filling onto the cold crust and spread it level with an offset spatula, pressing gently into the corners to remove large air pockets. Spoon the completely cooled blueberry topping over the surface and spread it into an even layer without digging into the filling. Cover the pan without allowing the wrap or lid to touch the topping. Refrigerate on a level shelf for at least 6 hours, preferably overnight. The center should feel firm under light pressure and should not ripple when the pan is nudged.

Run a thin knife along any exposed pan edges, then use the parchment overhang to lift the fully chilled cheesecake onto a cutting board. For 16 defined bars, mark four equal rows in each direction before cutting. Dip a sharp chef's knife in hot water, wipe it completely dry, and press straight down through each cut rather than sawing. Clean and rewarm the blade between slices. Serve the bars cold or let them stand for 5 to 10 minutes for a slightly softer, creamier texture.