Heat the oven to 375°F (190°C). Line a 12-cup muffin pan with paper liners or grease each cavity thoroughly.

Beat the softened cream cheese, granulated sugar, egg yolk, and vanilla extract until smooth. Set the filling aside while preparing the batter.

Whisk the flour, granulated sugar, baking powder, and fine salt together in a large bowl.

In a separate bowl, whisk the melted and slightly cooled butter, eggs, milk, and vanilla extract until combined.

Add the wet mixture to the dry mixture and stir only until no dry flour remains. A few small lumps are fine, and overmixing should be avoided.

Gently fold in the blueberries, taking care not to crush the fruit or overwork the batter.

Spoon about half of the batter into the muffin cups. Add approximately 1 tablespoon of cream cheese filling to the center of each cup, then cover with the remaining batter so the cups are about three-quarters full.

Bake for 20–25 minutes, until the tops are lightly golden. Test the muffin portion with a toothpick rather than piercing the cream cheese center; the toothpick should come out clean from the cake.

Cool the muffins in the pan for 5–10 minutes, then transfer them to a wire rack. Refrigerate leftovers because of the cream cheese filling.