Preheat oven to 350 degree F. Spray donut pan with non stick baking spray. Set aside.
In a large mixing bowl, begin making the donut batter. Combine the sugar with butter and beat until blended. Add in the egg. Mix in the milk, flour, baking powder, cinnamon, and salt. Beat until combined (about 1-2 minutes).
Once it's all combined, spoon it into a large ziploc bag and snip off the corner. Then squeeze into each donut cavity, evenly distributing the batter between donuts.
For the streusel, combine flour, brown sugar, cinnamon and melted butter with a fork. Spoon over batter.
Bake for 10-13 minutes, until lightly browned. Remove from oven and cool in pan about 10 minutes. Remove and cool completely on wire rack.
For the glaze, whisk together the sugar, milk, and cinnamon until smooth. Drizzle over cooled donuts and allow to set, about 5 minutes. ENJOY!