Preheat your oven to 350°F (175°C) and prepare a 6-mold donut pan with non-stick cooking spray.

In a small bowl, combine granulated sugar, all-purpose flour, and ground cinnamon. Cut in the cold butter until the mixture resembles coarse crumbs and chill.

In a large bowl, whisk together flour, brown sugar, salt, and baking powder. In a separate bowl, mix sour cream, melted butter, and egg. Fold wet into dry until just combined.

Spoon the batter into the prepared molds, filling each about three-quarters full and sprinkle with crumb topping.

Bake for 10-11 minutes until golden brown and springy when pressed.

Cool in the pan for 10 minutes then transfer to a cooling rack.

For the glaze, whisk together confectioners' sugar, milk, and vanilla until smooth, adjusting milk as needed.

Once cooled, drizzle the glaze over each donut and serve warm.