Preheat & prep - Heat oven to 350℉ . Grease donut pan.
Mix wet - Whisk pumpkin, sugars, eggs, butter, buttermilk, and vanilla until smooth.
Mix dry - In another bowl, whisk flour, baking powder, baking soda, salt, and pumpkin pie spice.
Combine - Fold dry into wet until just combined (don't overmix).
Fill pans - Spoon or pipe batter into the donut pan, filling each about 3/4 full.
Make streusel - Mix flour, brown sugar, cinnamon, and salt. Cut in butter until crumbly. Sprinkle over the donuts.
Bake - 14 to 16 minutes or until a toothpick comes out clean. Cool 5 minutes in pan, then remove from pan and cool on a rack.
Glaze drizzle - Whisk together glaze ingredients until smooth. Drizzle over cooled donuts with a spoon or piping bag.