01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl until evenly distributed.
03 - In a separate bowl, combine melted butter, milk, eggs, lemon juice, lemon zest, and vanilla. Whisk until completely smooth.
04 - Fold wet ingredients into dry mixture using a spatula. Mix just until combined—batter should remain slightly lumpy. Avoid overmixing.
05 - Beat cream cheese with sugar, egg yolk, and lemon zest until smooth and creamy.
06 - Place 2 tablespoons batter in each muffin cup. Add 1 heaping teaspoon cream cheese filling to center, then top with remaining batter, filling cups 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 the edge—avoid the cream cheese center.
09 - Let muffins rest in tin for 5 minutes. Transfer to wire rack and cool completely before serving.