Preheat your oven to 400°F (200°C). Line a 12-cup standard muffin pan with paper liners. Give them a quick spray with non-stick cooking spray for extra insurance against sticking.

In a small bowl, gently toss the diced peaches with 1 tablespoon of granulated sugar, cornstarch, and 0.25 teaspoon of ground cinnamon. Set aside.

In another small bowl, combine the 0.25 cup all-purpose flour, light brown sugar, and 0.25 teaspoon of ground cinnamon. Add the cold, cubed butter and use your fingertips or a pastry blender to cut the butter into the dry ingredients until coarse crumbs form. Don't overmix; you want some pea-sized pieces of butter. Set aside.

In a large mixing bowl, whisk together the 1.5 cups all-purpose flour, 0.5 cup granulated sugar, baking powder, and salt until well combined.

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

Pour the wet ingredients into the dry ingredients. Stir gently with a spatula just until *barely* combined. A few lumps are perfectly fine – overmixing will lead to tough muffins, and we definitely don't want that!

Divide the muffin batter evenly among the 12 prepared muffin cups. Spoon a small amount of the peach mixture over the top of each muffin. Finally, sprinkle a generous amount of the streusel topping over the peaches.

Bake for 5 minutes at 400°F (200°C), then, without opening the oven, reduce the temperature to 375°F (190°C) and bake for another 17-20 minutes, or until the muffins are golden brown and a wooden skewer or toothpick inserted into the center comes out clean.

Let the muffins cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature!