Preheat the oven to 425º F. In a large bowl, add your dry ingredients and whisk to blend them all together.
In a smaller bowl, add your wet ingredients and mix well. I do NOT recommend making gluten free muffins with a standing mixer. A standing mixer will over-mix your batter resulting in dense muffins.
Chop up your peaches into small pieces. Note it is optional if you want to peel your peaches.
Pour the wet ingredients into the dry ingredients and add the chopped peaches. Mix until the batter is "just barely mixed." This is very important if you want light and fluffy muffins!
In a small bowl, add the brown sugar, gluten free flour, and cinnamon. Whisk to blend.
Add the very cold butter and use a pastry blender to cut the butter into the dry streusel topping ingredients.
Add the muffin batter to your muffin liners. I highly recommend using parchment paper liners so your muffins do not stick to the paper! Fill each about ¾ full. Sprinkle the streusel topping on the muffins if you like.
Bake at 425º F for 5 minutes, then reduce the heat of the oven down to 350º F. The high temperature in the first 5 minutes will help give your muffins a domed top. If you don't want much dome lift, bake the whole time at 350º F. Bake for 20-25 minutes until done.
To test for doneness, insert a toothpick into the center of a muffin. If the toothpick comes out clean, it is done baking. If you see batter or crumbs, the muffins need to bake longer.