Preheat your oven to 400°F (205°C). Line a 12-cup muffin pan with paper liners.
In a large bowl, whisk melted butter and sugar together until smooth and slightly fluffy—about 2 minutes.
Add eggs one at a time, whisking well after each. Then mix in the vanilla, sour cream, and milk until combined.
In a separate medium bowl, whisk together flour, baking powder, and salt.
Gently fold dry ingredients into the wet mixture using a spatula. Be careful not to overmix—just until you don’t see streaks of flour.
Fold in the blueberries gently. If you’re using frozen berries, toss them with a teaspoon of flour first to prevent bleeding.
Scoop the batter evenly into the lined muffin cups, filling each about 3/4 of the way.
Make the streusel: Stir together melted butter, sugar, flour, and cinnamon until crumbly. Sprinkle a generous amount over each muffin.
Make the cheesecake filling: In a small bowl, beat cream cheese, sugar, flour, milk, and vanilla extract until smooth and creamy.
Using a piping bag (or spoon), add 1–2 tablespoons of cheesecake filling into the center of each muffin. It’s okay if it sinks a little—it’ll bake beautifully!
Bake for 25–30 minutes, or until a toothpick comes out clean (a little cheesecake is fine, but no wet batter).
Let the muffins cool in the pan for 5–10 minutes, then transfer to a cooling rack. Allow to cool completely before eating so the cheesecake sets.