Preheat oven to 375°F (190°C). Line a muffin tin with paper liners or grease lightly.
In a small bowl, combine brown sugar, flour, and cinnamon for crumble topping. Cut in cold butter until crumbly. Set aside.
In a large bowl, whisk flour, baking powder, baking soda, salt, and cinnamon.
In another bowl, whisk melted butter, brown sugar, eggs, vanilla, and milk until smooth.
Add wet ingredients to dry ingredients and mix gently until combined.
Fold in diced peaches carefully.
Divide batter evenly into muffin cups.
Sprinkle crumble topping over muffins.
Bake 18–22 minutes until golden brown and toothpick comes out mostly clean.
Cool slightly before serving.
If desired, drizzle glaze over cooled muffins.
