1. Prepare Streusel: In a small bowl, whisk together the flour, sugar, cinnamon, and salt for the streusel. Using a pastry blender or your fingers, cut in the cold butter until the mixture is crumbly. Refrigerate for at least 30 minutes.

2. Preheat and Prep: Preheat the oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or spray with baking spray.

3. Combine Dry Ingredients: In a medium bowl, whisk together 1 ½ cups of flour, baking powder, salt, and baking soda.

4. Cream Butter and Sugar: In a large bowl with an electric mixer, beat the butter and 1 cup of sugar until light and fluffy, about 3-4 minutes. Add the egg and mix until combined.

5. Combine Wet Ingredients: In a small bowl, whisk together the milk, sour cream, and vanilla extract.

6. Make the Batter: Add the dry ingredients to the butter mixture in three additions, alternating with the wet mixture in two additions, beginning and ending with dry. Mix until just combined.

7. Add Peaches: Toss ¾ cup chopped peaches with 1 tablespoon flour. Gently fold into the batter.

8. Fill and Top: Divide batter into muffin cups. Top with remaining ¼ cup of peaches and spoonfuls of chilled streusel.

9. Bake: Bake at 350°F for 20–25 minutes or until a toothpick comes out clean.

10. Cool: Let muffins cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.