Preheat oven to 350 degrees Fahrenheit. Spray two 6-count donut pans with non-stick baking spray.
Combine flour, baking powder, pumpkin spice, salt, and Domino® Golden Sugar in a medium bowl and stir together with a hand whisk.
In a large bowl, whisk together pumpkin puree, sour cream, vegetable oil, eggs, egg yolk, and vanilla until smooth.
Next, spoon half of the flour mixture into the pumpkin puree mixture. Stir with a rubber spatula until combined and you no longer see flour streaks. Then repeat with the other half of the flour mixture.
Transfer batter to large piping bag and pipe into pans. Fill donut wells ¾ full. Gently bang pan on counter to level batter.
Place donut pans on middle rack and bake for 14-16 minutes until lightly golden and bounce back when gently pressed with finger.
Place pans on cooling rack for 5 minutes.
Carefully tilt pan to let donuts slide onto cooling rack to completely cool.
Meanwhile, mix Domino® Golden Sugar and pumpkin spice in a large shallow bowl.
Take a single donut and brush melted butter onto one side with pastry brush. Dip into sugar coating and repeat with other side. Place donut onto serving dish.
Repeat coating with remaining donuts.