Preheat your 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, mix melted butter, egg, milk, and vanilla until well combined.
Gently fold the wet ingredients into the dry ingredients until just combined.
Carefully fold in the blueberries.
In a separate small bowl, blend cream cheese, sugar for filling, and egg yolk until smooth.
Fill each muffin cup halfway with batter; add a teaspoon of cream cheese mixture on top; then cover with remaining batter.
Bake for 18–22 minutes until golden brown and a toothpick comes out clean.
Allow them to cool slightly before transferring to a wire rack.
