Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or lightly grease the cups.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
In a separate large bowl, whisk together the melted butter, sugar, eggs, vanilla extract, and sour cream until smooth.
Gradually stir the dry ingredients into the wet ingredients until just combined.
Gently fold in the diced peaches.
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full.
In a small bowl, combine the flour, brown sugar, and cinnamon for the streusel. Cut in the cold butter until coarse crumbs form.
Sprinkle the streusel evenly over each muffin.
Bake for 20–25 minutes, or until a toothpick inserted into the center comes out clean.
Cool the muffins for about 10 minutes. If desired, whisk together the glaze ingredients and drizzle over the cooled muffins before serving.