Preheat the oven to 375°F (190°C) and prepare the muffin tin by greasing it or lining it with cupcake liners.

In a large bowl, whisk together the all-purpose flour, sugar, baking powder, salt, and cinnamon.

In a separate bowl, whisk the milk, oil, egg, and vanilla extract until smooth and frothy.

Gently fold the wet mixture into the dry ingredients, then add the diced peaches.

In a small bowl, combine the cold butter, brown sugar, and flour until the mixture resembles coarse crumbs.

Fill each muffin cup with batter until about two-thirds full and sprinkle generously with the streusel topping.

Bake for 20 to 25 minutes, until golden brown and a toothpick inserted in the center comes out clean.

Let the muffins cool in the tin for about 5 minutes, then transfer them to a wire rack to cool completely.