01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease thoroughly with cooking spray.
02 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt until evenly distributed.
03 - In a separate bowl, whisk melted butter, milk, eggs, lemon juice, lemon zest, and vanilla extract until fully blended and smooth.
04 - Pour wet ingredients into dry mixture. Gently fold together with a spatula until just combined—batter should remain slightly lumpy. Avoid overmixing to prevent tough muffins.
05 - In a small bowl, beat softened cream cheese with sugar, egg yolk, and lemon zest using a fork or electric mixer until completely smooth and creamy.
06 - Spoon approximately 2 tablespoons batter into each muffin cup. Place a heaping teaspoon of cream cheese filling in the center of each cup. Cover filling with remaining batter, filling cups to about 3/4 full.
07 - Sprinkle coarse sugar evenly over muffin tops. Bake for 18 to 22 minutes until golden brown. Test doneness by inserting toothpick into the edge (avoiding the cream cheese center)—it should come out clean.
08 - Let muffins rest in the tin for 5 minutes to set. Transfer to a wire rack and cool completely before serving or storing.