01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease thoroughly with cooking spray.
02 - In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt until well combined.
03 - In a separate bowl, combine melted butter, milk, eggs, lemon juice, lemon zest, and vanilla extract. Whisk until fully blended and smooth.
04 - Gently fold wet ingredients into dry mixture using a spatula. Mix just until combined—do not overmix as this will result in dense muffins.
05 - In a small bowl, beat softened cream cheese with sugar, egg yolk, and lemon zest until smooth and creamy.
06 - Spoon about 2 tablespoons of batter into each muffin cup. Place 1 heaping teaspoon of cream cheese filling in center, then cover with remaining batter. Fill each cup about ¾ full.
07 - Sprinkle tops generously with coarse sugar for a sweet crunch, if desired.
08 - Bake for 18–22 minutes until golden brown. Test doneness by inserting toothpick into the edge (avoid center cream cheese filling)—it should come out clean.
09 - Allow muffins to cool in the tin for 5 minutes before transferring to wire rack to cool completely.