Preheat your oven to 375℉ and prepare the muffin baking pan by adding paper muffin liners. I also recommend spraying the liners with cooking oil spray to help remove the muffins after baking.
Grab a large mixing bowl, and add flour, granulated sugar, brown sugar, baking powder, baking soda, and salt. Use a whisk (or mixing spoon if whisk is unavailable) to thoroughly mix all of these ingredients together. Set this bowl of dry ingredients aside.
In a separate mixing bowl, add in the 1 stick of melted butter, buttermilk, yogurt, egg and vanilla extract and mix until well combined. You can use a handheld mixer or stand mixer to mix.
Combine the wet ingredients and dry ingredients by slowly pouring the bowl of dry ingredients from Step #2 into the bowl of wet ingredients. Mix until just combined, do not overmix.
Now it’s time for the star of the show! Gently pour and fold in the diced peaches.
Grab your prepared muffin baking pan and divide the batter evenly between the muffin cups. I recommend filling each muffin cup anywhere between ⅔-¾ of the way full. You can sprinkle the tops with a bit of brown sugar, if desired.
Bake at 375℉ for 18-22 minutes or until a toothpick inserted into the center of one of the muffins comes out clean.
Carefully remove the muffins from the oven and allow to cool for 5-10 minutes.