Preheat the oven to 425F degrees. Prepare two muffin tins by either lining with muffin papers or spraying with non-stick cooking spray. You'll end up with about 14-16 muffins total.
To make the streusel topping, combine all the ingredients together in a small bowl. Set aside.
To make the muffins, first whisk together the flour, baking powder, baking soda and salt in a medium bowl.
In a large bowl using a stand or hand-held electric mixer on medium speed beat together the butter and sugars until light and fluffy in texture. Add in the eggs and vanilla and continue beating until well combined. Turn the mixer down to low and beat in the sour cream and milk.
Turn off the mixer and gently stir in the flour mixture using a large rubber spatula or wooden spoon. Once about ½ combined, add in the peaches and continue stirring the mixture until the flour is almost combined. There shouldn't be large streaks of flour, but a few lumps in the batter is ok.
Spoon the muffin batter into the prepared muffin tins filling each muffin cavity to the very top. Crumble the streusel in your fist, then sprinkle over the top of each muffin.
Bake for 5 minutes at 425F degrees, then without removing the muffins from the oven, turn the oven down to 375F degrees and continue baking for 8-10 minutes or until an inserted toothpick comes out clean. Allow to cool for 10 minutes before removing from the tin and continue cooling on a wire rack