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 blended.
03 - In a separate bowl, combine melted butter, milk, eggs, lemon juice, lemon zest, and vanilla extract. Whisk until smooth and fully incorporated.
04 - Gently fold the wet ingredients into the dry mixture using a spatula. Mix until just combined—do not overmix to maintain tender texture.
05 - In a small bowl, blend cream cheese with sugar, egg yolk, and lemon zest using a fork or electric mixer until smooth and creamy.
06 - Spoon about 2 tablespoons of muffin batter into each prepared cup. Place 1 heaping teaspoon of cream cheese filling in the center, then cover with remaining batter. Fill each cup approximately ¾ full.
07 - Sprinkle coarse sugar evenly over the tops of each muffin for a crunchy, sweet crust.
08 - Bake for 18–22 minutes until muffins are golden brown. Insert a toothpick into the edge (avoiding the cream cheese center)—it should come out clean.
09 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.