Follow these steps carefully for perfect Raspberry Swirl Cheesecake Bars:

1. Prepare the Pan and Oven:

• Preheat your oven to 325°F (160°C).
• Line an 8×8 inch (20×20 cm) square baking pan with parchment paper, leaving an overhang on two opposite sides. This overhang will act as “handles” to easily lift the bars out later. Lightly grease the bottom and sides of the pan before lining to help the paper stick.

2. Make the Graham Cracker Crust:

• In a medium bowl, combine the graham cracker crumbs and ¼ cup granulated sugar.
• Pour the melted unsalted butter over the crumb mixture. Stir with a fork until all the crumbs are evenly moistened. The mixture should resemble wet sand.
• Transfer the mixture to the prepared baking pan. Press it firmly and evenly onto the bottom of the pan using the bottom of a measuring cup or glass. Ensure the edges are packed slightly tighter.
• Bake the crust for 8-10 minutes, just until it’s lightly golden and set.
• Remove the crust from the oven and set it aside to cool slightly while you prepare the filling. Keep the oven on.

3. Prepare the Raspberry Swirl:

• In a small saucepan, combine the raspberries (fresh or frozen), 2 tablespoons granulated sugar, and 1 tablespoon lemon juice.
• Cook over medium heat, stirring occasionally, until the raspberries break down and the mixture begins to simmer. This usually takes about 5-8 minutes. Mash the raspberries gently with the back of a spoon as they cook.
• (Optional Thickener): If using cornstarch for a thicker sauce, whisk the cornstarch with 1 tablespoon of cold water in a small bowl until smooth. Pour this slurry into the simmering raspberry mixture and cook, stirring constantly, for another 1-2 minutes until the sauce thickens slightly.
• Remove the raspberry sauce from the heat. For a smoother swirl, you can strain the sauce through a fine-mesh sieve to remove the seeds. This is optional; some prefer the texture of the seeds. Let the sauce cool slightly while you make the filling.

4. Make the Cheesecake Filling:

• In a large bowl, using an electric hand mixer or a stand mixer fitted with the paddle attachment, beat the softened cream cheese on medium-low speed until completely smooth and creamy. Scrape down the sides and bottom of the bowl frequently to ensure there are no lumps. This step is crucial for a smooth cheesecake.
• Gradually add the ¾ cup granulated sugar and the pinch of salt. Continue beating on medium-low speed until just combined and smooth. Avoid overmixing.
• Add the room temperature eggs one at a time, beating on low speed just until each egg is incorporated. Do not beat vigorously after adding the eggs, as this incorporates too much air, which can lead to cracks.
• Stir in the room temperature sour cream (or Greek yogurt) and vanilla extract on low speed until just combined. The final batter should be smooth, thick, and creamy. Again, scrape the bowl to ensure everything is incorporated, but be gentle.

5. Assemble the Cheesecake Bars:

• Pour the cheesecake filling evenly over the slightly cooled graham cracker crust in the baking pan. Spread it into an even layer using an offset spatula or the back of a spoon.
• Dollop small spoonfuls of the cooled raspberry sauce randomly over the top of the cheesecake filling. You likely won’t use all the sauce; reserve any extra for serving.
• Using a thin knife, skewer, or toothpick, gently swirl the raspberry sauce into the cheesecake batter. Create marbled patterns by dragging the tool through the dollops. Be careful not to over-swirl, or the colours will become muddy. Aim for distinct ribbons of red against the white background.

6. Bake the Cheesecake Bars:

• Carefully place the pan into the preheated 325°F (160°C) oven.
• Bake for 30-40 minutes. The edges of the cheesecake should look set and slightly puffed, and the center should still have a slight wobble when you gently shake the pan. It should not be completely liquid, but it shouldn’t be firm either. Overbaking leads to a dry texture and potential cracks.
• Do not open the oven door frequently during baking.

7. Cool the Cheesecake Bars (Very Important!):

• Turn off the oven. Prop the oven door open slightly (using a wooden spoon handle works well) and let the cheesecake bars cool slowly in the turned-off oven for 1 hour. This gradual cooling process helps prevent cracking.
• After 1 hour, remove the pan from the oven and let it cool completely on a wire rack at room temperature (about another 1-2 hours).
• Once completely cool, cover the pan loosely with plastic wrap (don’t let it touch the surface) or foil and transfer it to the refrigerator.
• Chill for at least 4 hours, but preferably overnight (6-8 hours). Chilling is essential for the cheesecake to fully set and for the flavours to meld. Do not skip this step!

8. Slice and Serve:

• Once thoroughly chilled, use the parchment paper overhangs to carefully lift the entire cheesecake slab out of the pan and onto a cutting board.
• For clean cuts, use a large, sharp knife. Wipe the knife blade clean with a damp paper towel between each cut.
• Cut the slab into squares or bars (e.g., 9 large squares, 12 smaller bars, or 16 bite-sized squares).
• Serve chilled, plain or with desired toppings (see “How to Serve” section).