Preheat your oven to 375 degrees.
First in a large mixing bowl, or the bowl of a stand mixer, combine all the dry ingredients.
Next, in a medium bowl combine the pumpkin puree, brown sugar, butter, and sour cream. Mix well. Add the vanilla extract, egg, and sourdough starter. Mix until the mixture is well combined.
Once the wet ingredients are incorporated add the wet ingredients to the dry ingredients in the large bowl and mix just until combined. It’s okay if there are still a few lumps you just want to be sure not to overmix the batter.
Next, grease your donut pan if necessary. I have a silicone donut pan so I usually do not grease it.
Place the batter in a zip top bag or pastry bag and with a wide end (about ½”-1”) pipe the donut batter into the donut pan. I use a kitchen shears or my fingers to pinch off the batter. Continue until all the donut spots are filled to the top of the pan or a little below.
Place the donuts in the oven and bake for 16-20 minutes or until they spring back when touched lightly and are slightly brown.