Mix dry ingredients in a bowl until combined, then add 1 cup of diced peaches.
Add the wet ingredients to the dry ingredients and mix until combined.
Spray muffin tin with non-stick spray.
Divide the batter into 12 parts, then top with diced peaches and streusel topping.
Bake 15 to 18 minutes.
To make the streusel topping
Combine melted butter, flour, and brown sugar in a bowl. Using your hand, mix the ingredients until they form a crumb texture. Running your fingers through the mixture will help to ensure the mixture does not clump.