01 - Preheat the oven to 350°F. Line a 9×9-inch baking pan with parchment paper, leaving an overhang on two sides for easy bar removal.
02 - In a medium bowl, combine the graham cracker crumbs, granulated sugar, and melted butter. Stir until the mixture resembles wet sand. Press firmly and evenly into the bottom of the prepared pan. Bake for 8 to 10 minutes until lightly golden. Set aside to cool slightly.
03 - In a large bowl, use an electric mixer to beat the softened cream cheese and granulated sugar on medium speed until completely smooth and free of lumps. Add the eggs one at a time, mixing well after each addition. Blend in the vanilla extract and sour cream until fully incorporated. Pour the batter evenly over the cooled crust.
04 - In a separate bowl, toss the diced peaches with granulated sugar, lemon juice, and cornstarch if using. Spread the peach mixture in an even layer over the cheesecake batter.
05 - Bake for 30 to 35 minutes, or until the center is set with only a slight jiggle when gently shaken. Remove from the oven and allow to cool to room temperature on a wire rack.
06 - Refrigerate the cheesecake bars for at least 3 hours, or until completely chilled and firm. Use the parchment overhang to lift the slab from the pan, then cut into 12 even bars using a sharp knife.
07 - In a chilled bowl, whip the heavy cream with powdered sugar using an electric mixer until soft peaks form. Drizzle or pipe over the cut bars just before serving.