Preheat oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
In another bowl, combine melted butter, egg, milk, and vanilla extract until smooth.
Fold wet ingredients into dry ingredients carefully, mixing just until combined.
Gently fold in the fresh blueberries to avoid crushing them.
In a small bowl, beat softened cream cheese, sugar, and egg yolk until smooth for the filling.
Spoon batter halfway into each muffin cup, add about a teaspoon of cream cheese filling, then top with remaining batter.
Bake muffins for 18 to 22 minutes until golden and a toothpick comes out clean.
Cool muffins slightly before transferring to a wire rack. Serve warm or at room temperature.