01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt until evenly distributed.
03 - In a separate bowl, combine melted butter, milk, eggs, lemon juice, lemon zest, and vanilla extract. Mix until fully incorporated.
04 - Gently fold wet ingredients into dry mixture until just combined. Avoid overmixing to prevent tough texture.
05 - Blend softened cream cheese with sugar, egg yolk, and lemon zest in small bowl until smooth and creamy.
06 - Spoon approximately 2 tablespoons batter into each muffin cup. Place 1 heaping teaspoon cream cheese filling in center, then cover with remaining batter until cups are 3/4 full.
07 - Sprinkle coarse sugar evenly over muffin tops if desired for crunchy texture.
08 - Bake for 18 to 22 minutes until golden brown. Test doneness by inserting toothpick into edge (avoiding cream cheese center)—it should come out clean.
09 - Let muffins cool in tin for 5 minutes before transferring to wire rack. Cool completely before storing.