Place apple cider in a small saucepan and bring to a boil over medium-high heat. Reduce heat to a simmer and cook until liquid has reduced to ½ cup. Set aside to cool to room temperature.
Pre-heat oven to 350 degrees and lightly spray a doughnut pan with cooking oil.
In the bowl of a stand mixer or in a large bowl with hand mixer, beat butter and sugar together until light and fluffy, about 3 minutes.
Add the eggs and mix to combine.
In a separate medium-sized bowl, whisk together flour, baking powder, cinnamon, and salt.
Stir dry ingredients into the butter sugar mixture, alternating with buttermilk and ½ cup apple cider reduction, until just combined. Take care not to over-mix.
Fold in diced macintosh apple.
Evenly distribute batter into prepared pan, filling cavities almost all the way. You can either spoon batter in or put it in a ziplock bag with the corner cut off and pipe it in.
Sprinkle a generous amount of streusel topping on top of each doughnut.
Bake doughnuts for 14-16 minutes, until doughnuts are starting to turn golden around edges and lightly spring back when touched.
Let cool in pan for 5 minutes before removing to a wire rack to cool completely.
Once cooled, drizzle glaze over the top.