Preheat oven to 375F (190C) and line a muffin tin with paper liners.
In a large bowl, stir together oil and butter.
Add sugar, egg, and egg white and whisk very well until combined.
Add buttermilk and vanilla extract and whisk to combine.
In a separate, medium-sized, bowl whisk together flour, cornstarch, baking powder, and salt.
Add all of the dry ingredients to the wet ingredients and use a spatula to gently fold ingredients together until mostly combined.
Add blueberries and stir until batter is just combined and blueberries are well incorporated. Set aside while you prepare the filling and topping.
Combine softened cream cheese, sugar, and vanilla extract and stir until smooth and well-combined. Set aside.
Whisk together flour, brown sugar, and salt.
Drizzle in melted butter and use a fork to toss ingredients together until combined, no flour/sugar remains unmoistened by the butter, and mixture is clumpy. Don’t over-mix or the streusel will become like a paste, you want it to be clumpy.
Divide batter into 15 muffin cups, filling each liner ⅔ of the way full.
Use a spoon to make a crater in the center of each muffin and portion about 2 teaspoons of your cream cheese mixture in the center.
Top with a heaping spoonful of remaining muffin batter and use a knife to gently swirl together the cream cheese mixture and muffin batter.
Sprinkle muffins evenly with streusel.
Transfer to 375F (190C) oven and bake for 25 minutes or muffins are baked through.
Allow to cool for 10 minutes or so before digging in (the cream cheese filling will be very hot!). Muffins will puff up in the oven, but due to the cream cheese filling they will deflate and maybe even cave in a bit (like a black bottom) as they cool. This is normal!
You can use frozen (do not thaw) blueberries, but often they leave purple streaks through your batter. Nothing wrong with that, just be aware.
Baking In Two Muffin Tins
This recipe makes 15 muffins. Since most muffins tin hold enough batter for only 12 muffins, you can either discard the excess or bake the remaining 3 muffins after the first 12 bake. The second batch will take less time, start checking at 17 minutes. I don’t recommend over-filling the muffin liners, as the batter tends to spread out and flatten on the top, which makes for unattractive muffins.
Store in an airtight container at room temperature for up to 48 hours or in the refrigerator for up to 5 days.