Preheat the oven to 350F. Grease 3 6-well doughnut pans with nonstick spray. Set aside.
In a large bowl, add brown sugar, melted butter, oil, buttermilk, egg, egg yolk, pumpkin, and vanilla. Whisk to combine.
1 cup light brown sugar, ¼ cup salted butter, ¼ cup neutral oil, ⅓ cup buttermilk, 1 large egg, 1 large egg yolk, 1 cup canned pumpkin puree, 1 teaspoon vanilla extract
Add flour, cinnamon, pumpkin pie spice, baking powder, baking soda, and salt. Stir until just combined.
1 ½ cups all purpose flour, 1 teaspoon ground cinnamon, 1 teaspoon pumpkin pie spice, 1 ½ teaspoons baking powder, ¼ teaspoon baking soda, ½ teaspoon kosher salt
Spoon batter into prepared donut pans. Fill them about ⅔ full. You may need to bake them in batches depending on how many donut pans you have.
Bake for about 12-15 minutes, or until the tops spring back when pressed lightly.
Remove donuts from the pans and transfer to a wire rack.
While donuts are warm, prepare the topping: In a small bowl, combine cinnamon and sugar.
½ cup granulated sugar, 1 teaspoon ground cinnamon
Lightly brush donuts with melted butter.
¼ cup salted butter
Generously coat the donuts in the cinnamon sugar mixture.
Notes
Butter: If using unsalted butter, increase the salt by 1/4 teaspoon.
Oil: I like to use olive oil, but any neutral oil, like vegetable oil or canola oil, will work.
Buttermilk: If you don't have buttermilk, you can easily make a buttermilk substitute.