Using a medium bowl, add sugars and cinnamon; stir until well combined.
Add the melted butter; stir until well combined.
Add the flour; stir until well combined. Set streusel aside.
For the Muffins
Preheat oven to 375°F.
Lightly spray a muffin tin with baking spray, or line muffin tin with paper liners.
Using a medium mixing bowl, add flour and baking powder; stir until well combined. Set mixture aside.
Using an electric mixer, cream together sugar, butter and salt until light and fluffy (~3-4 minutes on medium speed).
Using a medium mixing bowl, whisk together eggs, buttermilk, vanilla and oil. Pour liquid mixture into bowl of mixer in 2 additions, mixing well after each addition.
Add flour mixture and mix on low until well combined.
Fold in diced peaches.
Divide batter evenly into prepared muffin tin. (See note.)
Crumble the streusel mixture evenly on top of the muffins.
Bake for 36-38 minutes, or until a toothpick inserted into center of a muffin comes out clean.
Notes
I used larger tulip-style muffin liners, and I filled each liner up to the top of the pan. (Tulip-style liners stick up above the baking pan.) Regular liners are smaller, so I would recommend filling them only ~¾ full. This recipe will then yield more muffins (you’ll need 2 pans), but you’ll need to reduce the baking time.