Line a 12 tin muffin liner with 12 liners or 2 liners with 6 muffin tins in each (Your preference) and set aside. Melt the butter and prepare to measure the rest of your ingredients as it cools.
In a small bowl, make the streusel. Combine flour, white sugar, cinnamon, and cold cut up butter. Using your hands, mix it together until combined into a crumbly, streusel mixture. Set aside while you make the muffin batter.
1 tsp cinnamon, 6 tbsp butter, 1/3 cup white sugar, 3/4 cup all-purpose flour
In a medium-sized bowl, add the flour, baking powder, baking soda, cornstarch, and salt. Mix together and set aside.
In a large bowl, add oil, butter, buttermilk, egg, egg yolk, and vanilla. Whisk until well combined. It’s okay if it is clumpy.
1/2 cup butter, 3 tbsp oil, 1 egg, 1 egg yolk, 1 cup buttermilk*, 1 tbsp vanilla, 1 cup white sugar
Add in the dry ingredients and using a spatula, slowly fold until a soft muffin batter forms. Mix until JUST combined.
In the bowl with the diced peaches, add 1-2 tsps of flour and gently toss around until evenly coated. The light coating of flour will absorb any extra liquid it may have before adding it into the mixing bowl.
2 cups diced peaches
Add the peaches and mix in a few times.
Evenly divide the batter between the muffin liners.
Sprinkle streusel on top.
Bake the muffins at 425°F for 5 minutes, then turn down to 375°F and bake for an additional 15-17 minutes. The muffins are ready when they bounce back at touch or a toothpick inserted comes out clean.
Enjoy!!
Notes
*To create buttermilk, mix 1 cup of milk with 2 tbsp of lemon juice or vinegar. Allow it to sit on your counter for a minimum of 10 minutes. Make sure it is at room temperature when adding it into your bowl.