Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease thoroughly.
In a small bowl, combine 1/2 cup flour, brown sugar, and 1/4 teaspoon cinnamon. Cut in the cold butter using a pastry cutter or your fingertips until the mixture resembles coarse crumbs with pea-sized butter pieces. Set aside.
Whisk together flour, baking powder, salt, and 1/2 teaspoon cinnamon in a medium bowl.
In a separate large bowl, cream butter and sugar until light and fluffy, about 2-3 minutes. Beat in eggs one at a time, then vanilla extract.
Alternately add the flour mixture and milk to the butter mixture, beginning and ending with flour (three additions of flour, two of milk).
Gently fold in the diced peaches, being careful not to overmix.
Divide batter evenly among muffin cups, filling each about 3/4 full.
Sprinkle the streusel topping generously over each muffin.
Bake for 22-25 minutes until a toothpick inserted in the center comes out clean.
Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.