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 fully incorporated and smooth.
04 - Gently fold wet ingredients into dry mixture using a spatula. Stir just until combined—small lumps are fine. Do not overmix or muffins will become tough.
05 - In a small bowl, beat softened cream cheese with sugar, egg yolk, and lemon zest until completely smooth and creamy.
06 - Spoon approximately 2 tablespoons of batter into each muffin cup. Place a heaping teaspoon of cream cheese filling in the center of each. Divide remaining batter over tops, filling cups about ¾ full.
07 - Sprinkle coarse sugar evenly over muffin tops if desired. Bake for 18 to 22 minutes until golden brown and a toothpick inserted into the edge (avoiding the cream cheese center) comes out clean.
08 - Let muffins rest in the tin for 5 minutes before transferring to a wire rack. Cool completely before storing.