Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease lightly.

Prepare the crumb topping: In a small bowl, whisk together the flour, sugars, and cinnamon for the topping. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Set aside in the refrigerator.

Prepare the muffin batter: In a large bowl, whisk together the flour, sugar, baking powder, and salt for the batter.

In a separate medium bowl, whisk together the egg, melted butter, milk, and vanilla extract until well combined.

Pour the wet ingredients into the dry ingredients and mix gently until just combined. Do not overmix; lumps are fine. Gently fold in the diced peaches.

Divide the batter evenly among the 12 muffin cups, filling each about two-thirds full. Sprinkle a generous amount of the chilled crumb topping over the top of each muffin.

Bake for 5 minutes at 400°F, then reduce the oven temperature to 375°F (190°C) without opening the door, and continue baking for 18-20 minutes more, or until a toothpick inserted into the center comes out clean.

Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.