Preheat an oven to 325 F. Grease a 12-cup bundt pan really well and dust with flour.
Add the apple cider to a small saucepan over medium-low heat. Simmer and reduce the cider by half, so that there's only 1 cup left. Cool to room temperature.
Whisk together the flour, cinnamon, nutmeg, baking powder, baking soda, and salt together in a bowl and set aside.
In a large mixing bowl, whisk together the melted butter, sugars, and oil until smooth. Add the eggs, one at a time, whisking between each addition until smooth. Whisk in the sour cream or skyr, apple butter, and vanilla.
Add half of the flour mixture and whisk. Then, whisk in half of the cooled apple cider concentrate. Repeat with the remaining flour and cider and whisk until the batter is smooth. Pour the batter into the prepared bundt pan. Bake for 50-55 minutes, until a cake tester comes out coated with a couple of moist crumbs. Remove from the oven and cool the cake in the pan for 1 hour before inverting onto a plate or cake stand.
Brush the still-warm cake liberally with the 4 tablespoons of melted butter and sprinkle with the cinnamon sugar. For the sides, take some of the cinnamon sugar in your cupped hand and press onto the cake.