Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.

In a large mixing bowl, combine vegetable oil, melted butter, and granulated sugar, whisking until well-blended. Add the whole egg and egg white, beating until creamy and fluffy.

Pour in the buttermilk and vanilla extract, stirring gently until just combined.

In another bowl, whisk together flour, cornstarch, baking powder, and salt. Gradually fold this into the wet ingredients until just incorporated.

Toss blueberries with a touch of flour and fold gently into the batter.

For the cream cheese filling, beat cream cheese, sugar, and vanilla extract together until smooth.

Spoon a tablespoon of batter into each muffin cup, add the cream cheese filling, then top with more batter.

In a small bowl, mix brown sugar, flour, melted butter, and salt to create the streusel topping. Sprinkle this over each muffin.

Bake for 24-26 minutes until golden brown and a toothpick comes out clean.

Allow the muffins to cool in the pan for about 10 minutes before transferring to a wire rack.