Step 1: Prepare the Pan and Preheat Oven

• Preheat your oven to 350°F (175°C).
• Grease and flour a 9×13 inch baking pan. Alternatively, line the pan with parchment paper, leaving an overhang on two sides for easy lifting later. This is highly recommended.

Step 2: Make the Streusel Topping

• In a medium bowl, whisk together the ¾ cup flour, ½ cup granulated sugar, ¼ cup brown sugar, cinnamon (if using), and pinch of salt.
• Add the cold, cubed butter to the flour mixture.
• Using a pastry blender, your fingertips, or two forks, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
• Place the bowl of streusel topping in the refrigerator to keep cold while you prepare the rest of the cake.

Step 3: Make the Cream Cheese Filling

• In a separate medium bowl, combine the softened cream cheese and ¼ cup granulated sugar.
• Using an electric hand mixer or a stand mixer fitted with the paddle attachment, beat on medium speed until smooth and creamy, scraping down the sides of the bowl as needed.
• Add the egg yolk and 1 teaspoon vanilla extract. Beat again until just combined and smooth. Do not overmix. Set aside.

Step 4: Prepare the Cake Batter

• In a large bowl, whisk together the 2 ½ cups all-purpose flour, baking powder, baking soda, and ½ teaspoon salt. Set aside.
• In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using an electric hand mixer, cream the softened ¾ cup unsalted butter and 1 ½ cups granulated sugar together on medium-high speed until light and fluffy (about 3-4 minutes).
• Reduce the speed to medium and add the eggs one at a time, beating well after each addition. Scrape down the sides and bottom of the bowl. Beat in the 1 teaspoon vanilla extract.
• In a small bowl or measuring cup, stir together the sour cream (or Greek yogurt) and milk until combined.
• Reduce the mixer speed to low. Add the dry ingredients (flour mixture) in three additions, alternating with the sour cream/milk mixture in two additions, beginning and ending with the dry ingredients. (Dry -> Wet -> Dry -> Wet -> Dry).
• Mix only until just combined after each addition. Be very careful not to overmix the batter at this stage, as it can lead to a tough cake. Stop mixing as soon as no large streaks of flour remain. A few small lumps are okay.

Step 5: Gently Fold in Blueberries

• If desired, gently toss the fresh or frozen blueberries with 1 tablespoon of the measured flour mixture in a small bowl. This can help prevent them from sinking.
• Gently fold the blueberries into the cake batter using a rubber spatula. Mix minimally, just enough to distribute them evenly.

Step 6: Assemble the Cake

• Spread half of the cake batter evenly into the prepared 9×13 inch baking pan.
• Dollop the prepared cream cheese filling evenly over the batter layer. You can gently spread it with an offset spatula or the back of a spoon, but it doesn’t need to be perfectly smooth or reach the absolute edges. Leaving some gaps is fine.
• Carefully spread the remaining cake batter over the cream cheese layer. It might be a bit tricky; use dollops and gently spread to cover the filling as best as possible. It doesn’t have to be perfect, as the streusel will cover it.
• Retrieve the chilled streusel topping from the refrigerator. Sprinkle it evenly over the top layer of batter, covering the entire surface.

Step 7: Bake the Cake

• Place the pan in the preheated oven.
• Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the cake portion (avoiding the cream cheese layer and blueberries) comes out clean or with only moist crumbs attached. The top should be golden brown, and the edges should be set.
• Note: Baking time can vary depending on your oven and pan material. Start checking around the 50-minute mark. If the top is browning too quickly, you can loosely tent the cake with aluminum foil for the remainder of the baking time.

Step 8: Cool the Cake

• Once baked, carefully remove the cake from the oven and place the pan on a wire rack.
• Let the cake cool completely in the pan before slicing and serving. This is important! Cooling allows the structure to set properly and prevents the cream cheese filling from being too runny. Cooling can take 2-3 hours at room temperature.

Step 9: Slice and Serve

• If you used parchment paper, use the overhangs to lift the cooled cake out of the pan onto a cutting board.
• Slice into squares and serve. Enjoy!