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 combined.
03 - In a separate bowl, mix melted butter, milk, eggs, lemon juice, lemon zest, and vanilla extract until well blended.
04 - Fold wet ingredients into dry mixture until just combined. Do not overmix—some lumps are fine.
05 - Blend softened cream cheese with sugar, egg yolk, and lemon zest in a small bowl 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, filling cups ¾ full.
07 - Sprinkle coarse sugar on tops if desired. Bake for 18-22 minutes until golden and a toothpick inserted into the edge (avoiding the cream cheese center) comes out clean.
08 - Let muffins rest in tin for 5 minutes, then transfer to wire rack to cool completely.