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.
04 - Gently fold wet ingredients into dry mixture until just combined. Stop mixing as soon as flour disappears—do not overmix.
05 - In a small bowl, beat cream cheese with sugar, egg yolk, and lemon zest until smooth and creamy.
06 - Spoon about 2 tablespoons batter into each muffin cup. Place 1 heaping teaspoon cream cheese filling in center, then cover with remaining batter. Fill each cup about ¾ full.
07 - Sprinkle coarse sugar evenly over muffin tops, if desired.
08 - Bake for 18–22 minutes until golden brown. Test doneness by inserting toothpick into edge (avoid cream cheese center)—it should come out clean.
09 - Let muffins cool in tin for 5 minutes, then transfer to wire rack to cool completely.