01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease thoroughly with nonstick spray.
02 - In a large mixing 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. Whisk until completely blended.
04 - Gently fold wet ingredients into dry mixture until just combined. Avoid overmixing to ensure tender muffins.
05 - In a small bowl, blend cream cheese with sugar, egg yolk, and lemon zest until smooth and creamy.
06 - Spoon about 2 tablespoons batter into each muffin cup. Drop 1 heaping teaspoon cream cheese filling in center, then cover with remaining batter, filling cups ¾ full.
07 - Sprinkle coarse sugar evenly over muffin tops if desired for added texture and sweetness.
08 - Bake for 18-22 minutes until golden brown. A toothpick inserted into the edge (avoiding center filling) should come out clean.
09 - Let muffins cool in tin for 5 minutes before transferring to wire rack. Cool completely before storing.