Preheat the oven to 350ºF (177ºC), Spray an 8x4-inch loaf pan with nonstick spray. Place a piece of parchment paper that’s cut to the width of the pan, lengthwise in the pan, allowing overhang. Spray the parchment paper with nonstick spray again and place another piece of parchment paper across the pan and press down so the parchment sticks to the other piece. Spray the parchment with nonstick spray and set aside.
In a small saucepan, heat the apple cider over medium heat, stirring occasionally, until it comes to a simmer. Continue to simmer, stirring occasionally, until it is reduced by about half, around 10 minutes. Remove from the heat and allow to cool for 5 minutes. Pour ¼ cup into a heat proof measuring cup or bowl and set aside for topping the cake later. Pour remaining ¾ cup apple cider reduction into a separate bowl and whisk in the sour cream and vanilla, whisking well until combined. Set aside.
In a small bowl, whisk together flour, baking soda, salt and cinnamon. Set aside. In the bowl of a stand mixer, cream together butter and sugar until light and fluffy. Add eggs one at a time and continue mixing on medium-high for 3 minutes until light in color and creamy. Scrape sides and bottom of the bowl with a spatula. With the mixer on low, add in the flour a little at time, alternating with the apple cider mixture, beginning and ending with the flour, scraping the sides and bottom of the bowl as needed. Do not overmix.
Scrape the batter into the prepared loaf pan, set on a baking sheet and bake for 1 hour. Remove from the oven and with a toothpick, poke holes all over the top of the cake.