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

Prepare the crumble topping: In a small bowl, combine the 1/2 cup flour, brown sugar, and cinnamon. Cut in the cold, cubed butter using your fingers or a pastry blender until the mixture resembles coarse crumbs. Set aside in the refrigerator.

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

In a separate medium bowl, whisk together the melted butter, egg, milk, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix; a few lumps are okay.

Gently fold in the diced peaches until evenly distributed throughout the batter. Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full.

Sprinkle the reserved crumble topping generously over the top of each muffin cup. Bake for 5 minutes at 400°F, then immediately reduce the oven temperature to 375°F (190°C) and continue baking for another 18-20 minutes, 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.