Preheat your oven to 350°F and grease a donut pan with non-stick spray.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.

In a large bowl, cream together the softened butter, white sugar, and brown sugar until fluffy.

Add the eggs and vanilla extract, and mix until pale and fluffy.

Mix in the sour cream and buttermilk until combined.

Gradually add the dry ingredients to the wet ingredients, mixing until well combined.

Transfer the donut batter to a piping bag and pipe it into the prepared donut pan.

Bake for 12-14 minutes until the edges are lightly golden.

Let the donuts cool in the pan for 2 minutes before transferring them to a cooling rack to cool completely.

In a small bowl, combine the granulated sugar and ground cinnamon.

Once the donuts are cooled, brush each one with melted butter and roll them in the cinnamon sugar mixture.

Now, they're ready to be enjoyed!