Step 1: Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift the bars later. Set aside.
Step 2: In a medium bowl, combine the crushed digestive biscuits/graham crackers, melted butter, and optional sugar. Mix until well combined and the crumbs are evenly moistened.
Step 3: Press the mixture firmly into the bottom of the prepared baking pan to form an even crust. Place in the refrigerator to chill while you prepare the filling.
Step 4: In a large bowl, using an electric mixer, beat the room temperature cream cheese until smooth and creamy, about 2-3 minutes.
Step 5: Gradually add the powdered sugar to the cream cheese, beating until well combined and lump-free. Stir in the vanilla extract.
Step 6: In a separate, chilled bowl, whip the cold heavy whipping cream with an electric mixer until stiff peaks form. Be careful not to overbeat.
Step 7: Gently fold the whipped cream into the cream cheese mixture until just combined and no streaks of cream remain. Avoid overmixing.
Step 8: If adding blueberries directly to the filling, gently fold in about half a cup now. Pour the cream cheese mixture over the chilled crust and spread evenly with a spatula.
Step 9: If desired, sprinkle the remaining blueberries over the top of the cheesecake filling. Gently press them down slightly.
Step 10: Cover the pan loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, until the cheesecake is firm and set.
Step 11: Once chilled, use the parchment paper overhang to lift the entire cheesecake out of the pan. Slice into bars and serve cold. Store any leftovers in an airtight container in the refrigerator.