Preheat oven to 350F degrees. If your donut pans are not nonstick, spray them with cooking spray.
Place the sour cream, applesauce, eggs, vanilla, oil, butter, sugar, and salt in a bowl. Whisk together.
Mix the flour and baking soda together, then add to the sour cream mixture. Stir until well combined.
Fill the nonstick donut pans with batter. I use a soup spoon to fill each donut opening. It takes two full soup spoons to fill each hole.
Bake in 350F oven for 12 minutes. The donuts will be golden brown and the top will be springy to the touch.
Remove the pans from the oven, and let cool for at least 5 minutes before turning the donuts out onto a wire cooking rack. Be careful, as they tear easily when they're hot.