Preparation Steps

• Line a 9x9-inch square pan with parchment, leaving an overhang for easy removal. Place the pan in the fridge while you assemble the crust.
• Make the crust: pulse the biscuits in a food processor or smash in a zip-top bag until very fine. Mix the crumbs with the melted butter and a pinch of salt until evenly moistened. Press the mixture firmly into the bottom of the chilled pan in an even layer. Return the pan to the refrigerator to firm while you make the filling (about 15 minutes).
• Prepare the lemon curd: in a medium bowl whisk together the cornstarch, brown sugar, lemon zest (from two lemons), whole eggs and yolks, and 1/2 cup fresh lemon juice until smooth and no lumps remain. Transfer the mixture to a medium saucepan.
• Cook the curd over low to medium-low heat, stirring constantly with a whisk. It will thicken after several minutes; once it coats the back of a spoon and you can draw a line through it, remove from heat. Immediately whisk in the cubed butter until glossy and incorporated. Pour the curd through a fine-mesh sieve into a shallow bowl to remove any bits, press plastic wrap directly on the surface, and chill in the refrigerator for at least 30 minutes to set.
• While the curd cools, make the cheesecake layer: chill a mixing bowl and whisk or beaters for a few minutes. Whip the cold heavy cream to soft peaks and set aside.
• In a separate large bowl, beat the room-temperature cream cheese and mascarpone with the powdered sugar, lemon zest (from 1 lemon), 1 tablespoon lemon juice, and vanilla until smooth and free of lumps. Fold the whipped cream into the cheese mixture gently with a spatula until uniform and airy.
• Spread the cheesecake filling evenly over the chilled crust, smoothing the top with an offset spatula. Return the pan to the refrigerator and chill the cheesecake layer for about 60 minutes, until it feels set to the touch.
• Remove the chilled lemon curd briefly from the fridge, give it a gentle stir if needed, then spoon it over the set cheesecake in an even layer. Smooth the curd and return the bars to the refrigerator for a final chill of at least 90 minutes (or overnight) so everything firms completely.
• To serve, lift the bars from the pan using the parchment overhang. Trim the edges for neat squares, garnish with thin lemon slices and extra zest if desired. For clean slices, wipe a knife with hot water and dry between cuts.