Preheat the oven to 375°F. Line a muffin tin with paper liners or grease with cooking spray.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, whisk together the melted butter, egg, milk, and vanilla extract.
Add the wet ingredients to the dry ingredients, and stir until just combined.
Gently fold in the blueberries.
In a small bowl, beat together the cream cheese, sugar, and egg yolk until smooth.
Spoon the muffin batter evenly into the prepared muffin cups, filling them halfway.
Drop a heaping teaspoonful of the cream cheese mixture on top of the batter in each muffin cup.
Cover the cream cheese with the remaining muffin batter, filling the muffin cups about 3/4 full.
Bake for 18-22 minutes, or until the muffins are golden and a toothpick inserted into the center comes out clean.
Cool the muffins in the pan for 5 minutes, then transfer them to a wire rack to cool completely.