Preheat oven to 350 degrees F. Liberally grease a standard bundt pan with cooking spray and set aside. Meanwhile, preheat a medium skillet over medium-heat.
Throw the butter, 1/4 cup brown sugar, one (1) tsp of the cinnamon and the apples into the skillet and saute until tender but not too soft, about 2-3 minutes. Remove from the heat and let cool.
Meanwhile, in a large bowl, beat the cake mix, pudding mix, oil, buttermilk, eggs, vanilla and remaining teaspoon of cinnamon with a handheld electric mixer until combined. Batter will be thick. Spread HALF of the batter into the prepared pan. Layer the apples evenly on top of the batter layer, including the juices from the apples in the skillet. Sprinkle the apples evenly with the remaining cup of brown sugar. Top with the remaining batter.
Bake for approx. 50-60 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Allow the cake to cool in the pan for about 30 minutes, then very carefully invert it onto a wire rack to cool completely.
Once cake has cooled, prepare your glaze: in a small bowl, whisk together the powdered sugar, apple cider and water until you've reached a pourable consistency. Pour the glaze over top of the cake and let it set, about 15 minutes, before cutting into slices.