Peel, core, and chop your apples. Combine with apple cider in a saucepan. Bring to a boil, then simmer over medium heat for 10-12 minutes, until apples are tender. The mixture may foam up when it boils.
Cool for 5 minutes, then puree, preferably with an immersion blender. Reducing the cider may take 35 minutes, from chopping to pureeing. Your puree may be liquidy.
Combine 1 cup (about 236 grams) of apple mixture with milk. The milk will curdle like buttermilk.
Cake Batter
Combine flour, baking powder and soda, salt, and nutmeg in a mixing bowl. Set aside.
Cream butter with sugar and brown sugar in stand mixer on medium speed using the paddle attachment for 3 minutes, until light and fluffy.
Add eggs, one by one, mixing for 1 minute after each addition.
Add oil and vanilla, and mix for another minute, until well mixed.
Alternate adding the flour and apple mixtures in batches, beginning and ending with the flour mixture. Scrape the bottom of the mixer bowl to make sure butter and sugar isn’t stuck there (like it was in mine). Mix for 10 more seconds to make sure everything is airy and light.
Pour batter into an extremely well greased 12-cup Bundt cake pan and bake at 350F for 35-50 minutes.
Cool cake in the pan for 10 minutes, then invert onto cake platter. I had a piece stick to my cake pan the first time I made it, so be sure you’ve greased it really well.
Cinnamon Sugar Topping
Combine sugar, and cinnamon. Coat cake with cinnamon sugar, patting it on the top and sides by hand.
Cool completely (or at least 45 minutes) before cutting. Store leftovers in an airtight container at room temperature for up to 3 days.
Notes
Apple flavor can be subtle in the cake, depending on the variety of apples used. Choose the most flavorful apples you can find. I like Pink Lady and Granny Smith; Galas weren't great. You can try to reduce the cider more if desired. I usually don't peel mine because they get blended with the cider.Use freshly grated nutmeg if at all possible. Its flavor is much better than pre-ground nutmeg.Don't shorten the mixing time. Mixing the batter for so long beats a lot of air into the batter and makes the cake incredibly fluffy. The high butter and oil content in this cake makes it very tender.Eat leftover apple cider mixture later with oatmeal, in smoothies, or on its own.My cake took 35 minutes exactly in my oven, even though my apple mixture was very thin; some people say the cake takes longer.Adapted from Serious Eats.