Step 1: Prepare the Cream Cheese Filling

1. In a small bowl, mix the cream cheese, sugar, and vanilla extract until smooth.
2. Set aside for filling the muffins later.

Step 2: Make the Muffin Batter

1. Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with liners or grease lightly.
2. In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
3. In another bowl, whisk together the melted butter, buttermilk, eggs, lemon zest, lemon juice, and vanilla extract.
4. Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Do not overmix.

Step 3: Assemble the Muffins

1. Spoon about 1 tablespoon of muffin batter into each liner.
2. Add 1 teaspoon of cream cheese filling on top of the batter.
3. Cover with another tablespoon of muffin batter, making sure the cream cheese is enclosed.
4. Repeat for all muffins.

Step 4: Bake the Muffins

1. Bake for 18-22 minutes, or until a toothpick inserted into the muffin (avoiding the cream cheese center) comes out clean.
2. Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Step 5: Make the Lemon Glaze (Optional)

1. In a small bowl, whisk together the powdered sugar, lemon juice, and zest until smooth.
2. Drizzle over cooled muffins before serving.