Preheat your oven to 425°F (220°C) and line a muffin pan with paper liners.

In a mixing bowl, combine flour, granulated sugar, light brown sugar, and ground nutmeg for the streusel topping. Add cold, cubed butter and mix until the mixture resembles coarse crumbs. Refrigerate.

In another bowl, whisk together all-purpose flour, baking powder, cinnamon, and salt.

In a large mixing bowl, beat melted butter and granulated sugar together until lightened in color. Incorporate eggs one at a time, then stir in vanilla extract.

Fold the dry ingredients into the wet mixture alternately with milk in three parts.

Gently fold in the diced peaches into the batter.

Divide the batter evenly among the muffin cups, filling each about 2/3 full, and sprinkle the chilled streusel topping over each muffin.

Bake at 425°F (220°C) for 5 minutes, then reduce to 350°F (175°C) and bake for an additional 15-18 minutes.

Let the muffins cool in the pan for 5 minutes before transferring to a wire rack for another 10 minutes.