Start by adding the apple cider to a saucepan over medium-low heat. Bring it to a simmer and let it simmer until it has reduced down to ½ cup, stirring occasionally. It will take some time, around 20-30 minutes. Start checking it has reached ½ cup at 20 minutes and add more time as necessary. Let the cider reduction cool before using it in the donut batter.
Preheat the oven to 180 degrees.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, nutmeg and cinnamon. Set aside the flour mixture.
In a large bowl, cream, the softened butter, granulated white sugar and light brown sugar together with an electric mixer on high speed until fluffy, about 2 minutes.
Add in the eggs and vanilla and mix at medium-high speed until pale and fluffy, about 1 minute.
Next, add in the buttermilk, sour cream and the reduced cider and mix at low speed until combined.
Add the dry ingredients to the wet ingredients and mix on low speed just until all is combined.
Grease a donut pan with nonstick spray. Transfer the donut batter to a piping bag and cut about ½ inch opening on the bottom. Pipe the batter into the donut pan until each well is a little more than ¾ full.
Bake the donuts for 12-13 minutes, or until a toothpick comes out clean from the center.
Let them cool for 5 minutes in the pan and then turn the pan over onto a wire rack to release the donuts. Let the outcool until cool enough to handle.