Preheat oven and line pan:
Heat oven to 350°F (175°C). Place paper liners in a muffin pan.Whisk dry ingredients together:
In a large bowl, combine flour, sugar, baking powder, baking soda, and salt with a whisk.Blend wet ingredients:
In another bowl, blend milk, melted butter, eggs, and vanilla until uniform.Combine wet and dry mixtures:
Pour wet mixture into dry mixture and stir gently until just incorporated.Fold in diced peaches:
Carefully fold the diced peaches into the batter.Fill muffin cups:
Divide batter among muffin cups, filling each about two-thirds full.Make and sprinkle topping:
In a small bowl, stir together cinnamon and brown sugar for the topping, then sprinkle over each muffin.Bake until golden:
Bake for 18-20 minutes until golden and a toothpick inserted in the center comes out clean.Cool muffins on rack:
Let muffins cool in the pan for 5 minutes, then move them to a wire rack to cool fully.