1. Prepare Crust and Oven:

• Preheat the oven to 160ºC / 325ºF (conventional oven).
• Prepare a 23 cm / 9 inch springform tin by lining the bottom with parchment paper. Do not grease the sides.
• In a food processor, add the digestive or graham crackers and 2 tablespoons granulated sugar. Blend until fine crumbs form.
• Melt the 75g of butter. Add the melted butter to the food processor and pulse until the crumbs are evenly moistened.
• Transfer the crumbs to the springform pan. Press the crumbs firmly down onto the bottom and up the sides of the pan using the bottom of a glass or measuring cup. Create an even crust.
• Bake the crust for 10 minutes. Remove from oven and let it cool on a wire rack while you prepare fillings. Keep the oven on.

2. Prepare Blueberries and Crumble:

• Blueberries: In a small bowl, gently mix together the fresh blueberries, 1 tablespoon granulated sugar, and 1 tablespoon all-purpose flour. Add the 2 teaspoons lemon juice and mix gently until no dry flour remains. Set aside.
• Crumble: In another medium bowl, combine the 110g all-purpose flour and 80g dark brown sugar. Melt the 70g butter and pour it over the flour mixture. Use a fork to mix until it forms a crumbly structure and no dry flour remains. Set aside.

3. Make the Cheesecake Filling:

• Ensure cream cheese and sour cream are at room temperature, and eggs are lightly beaten or ready to add.
• In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), beat the room temperature cream cheese on low speed for 1 minute until smooth.
• Add the 260g granulated sugar and mix on low speed for 1 minute until combined. Scrape down the sides of the bowl and mix for another 30 seconds.
• In a small bowl, whisk together the room temperature sour cream and the cornstarch until smooth. Add this mixture along with the vanilla extract to the cream cheese bowl. Mix on low speed until just combined.
• Add the room temperature eggs, two at a time, mixing on low speed only until each addition is just barely combined. Do not overmix once eggs are added.
• Scrape down the sides of the bowl and give the batter one final gentle mix on low speed or by hand to ensure everything is incorporated.

4. Assemble the Cheesecake:

• Pour the cheesecake batter over the cooled cookie crust in the springform pan.
• Gently scatter the prepared blueberry mixture evenly over the top of the cheesecake batter.
• Sprinkle the prepared crumble mixture evenly over the blueberries.

5. Prepare Water Bath and Bake:

• Method 1 (Pan-in-Pan): Boil water in a kettle. Place the filled springform pan inside a larger (30 cm/12 inch) cake pan. Place this larger pan inside another larger baking pan or roasting pan. Carefully fill the outermost roasting pan with the hot boiled water until it reaches about halfway up the sides of the middle cake pan (not the springform itself).
• Method 2 (Foil Wrap): Tightly wrap the outside of the springform pan with a triple layer of heavy-duty aluminum foil, ensuring it’s completely sealed against water leaks. Place the foil-wrapped pan directly into a large roasting pan. Carefully fill the roasting pan with about 1 inch of hot boiled water.
• Carefully place the entire water bath setup into the preheated oven.
• Bake for 1 hour and 20 minutes to 1 hour and 30 minutes.

6. Check Doneness and Cool:

• After the baking time, gently shake the pan. The edges of the cheesecake should be set, but the very center should still have a slight wobble.
• Turn off the oven. Prop the oven door open slightly with a wooden spoon. Let the cheesecake cool down slowly in the turned-off oven for 1 hour. This helps prevent cracking.
• Carefully remove the cheesecake (still in its springform) from the water bath setup. Remove foil if used. Place the cheesecake on a wire cooling rack.
• Let it cool completely to room temperature on the rack, about 1-2 hours more.

7. Chill Thoroughly:

• Once at room temperature, cover the cheesecake loosely with plastic wrap (avoid touching the top if possible) or place it in a cake carrier.
• Refrigerate for at least 6 hours, or preferably overnight, until completely cold and firm. Chilling is crucial for the best texture and flavor.

8. Serve:

• Once thoroughly chilled, run a thin knife around the edge of the cheesecake to loosen it from the springform ring. Carefully release and remove the ring.
• Slice the cheesecake with a large, sharp knife dipped in hot water and wiped clean between cuts for the neatest slices. Serve chilled.