Preheat oven to 425°F. Line and/or spray a 12-count muffin pan with nonstick spray, set aside. (The non-stick spray over the cupcakes liners allows for an easy removal at the end.) Mix all of the streusel ingredients together in a small bowl and set aside.
In a medium size bowl, whisk together the flour, baking soda, baking powder, and salt; set aside.
In a large bowl with a hand mixer or stand mixer with a paddle attachment, beat together the butter and both sugars on high until smooth and creamy. Beat in eggs one at a time, then beat in sour cream and vanilla.
Slowly beat in dry ingredients alternating with the milk until no flour pockets remain in the batter. Fold in 1¼ cups of peaches, reserving ¼ cup. Spoon batter into the liners all the way to the top. Add remaining ¼ cup of peaches on the tops of the muffins, then lightly pack streusel topping over the muffins.
Bake muffins for 5 minutes at 425°F, then reduce oven temperature to 350°F and bake for an additional 18-19 minutes. Let muffins cool for about 15 minutes before removing from the pan. Enjoy!
