Preheat oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, whisk together flour, baking powder, baking soda, and salt.
In a separate bowl, whisk melted butter, sugars, eggs, buttermilk, and vanilla until smooth.
Add wet ingredients to the dry ingredients and mix just until combined (do not overmix).
Gently fold in the diced peaches.
In another bowl, beat the cream cheese, sugar, and vanilla until smooth and creamy.
Fill each muffin cup ⅔ full with muffin batter. Add about 1 tablespoon of cream cheese filling to each center.
Use a toothpick to swirl the cream cheese into the batter with gentle figure-eight motions.
Bake for 18–22 minutes or until tops are golden and a toothpick comes out with a few moist crumbs.
Let cool in the pan for 5 minutes before transferring to a wire rack.