Preheat the oven to 350F (175C). To make the crumble, combine sugar, flour, and cinnamon in a small bowl. Cut in butter until it reaches a crumbly texture. Set aside.
In a large bowl whisk together the flour and brown sugar. Add baking powder, salt and whisk together.
In a medium bowl, whisk together the egg, sour cream, and melted butter until well combined. Add to the flour mixture and stir until just combined.
Grease a donut pan with non-stick cooking spray. Fill each section about 2/3 full with dough. Place the diced peaches in the center of each donut. Sprinkle with crumble topping.
Bake for 12 minutes or until a toothpick inserted comes out clean. Allow to cool in the pan for 5 minutes before transferring to a wire rack.
To make the glaze, whisk together powdered sugar, milk, and vanilla until smooth. If the glaze is too thick, add a little more milk (1 teaspoon at a time) until desired consistency is reached.
Drizzle the glaze over the donuts before serving.