Mix a generous pinch of kosher salt, ¼ cup (50 g) sugar, ½ tsp. cinnamon, and ¼ tsp. nutmeg in a small bowl, set aside.

Place a rack in middle of oven; preheat to 325°. Lightly butter an 8½x4½" or 9x5" loaf pan. Line with parchment paper, leaving overhang on both long sides. Bring 1½ cups apple cider to a boil in a medium saucepan over medium-high heat. Reduce heat and simmer until cider is reduced by half (about ¾ cup), 8–10 minutes. Pour 4 Tbsp. reduced cider into a small measuring glass or bowl and set aside. Transfer remaining reduced cider to a small bowl and let cool 5 minutes. Stir in ½ cup sour cream and 1 tsp. vanilla extract and set aside.

Melt 8 Tbsp. unsalted butter in saucepan over low heat. Let cool slightly.

Whisk 1¼ cups plus 2 Tbsp. (172 g) all-purpose flour, 2 Tbsp. (15 g) cornstarch, 1 tsp. baking powder, ½ tsp. baking soda, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ½ tsp. cinnamon, and ¼ tsp. nutmeg in a medium bowl to combine.

Vigorously whisk 2 large eggs, room temperature and ¾ cup (150 g) sugar in a large bowl until pale and frothy, about 2 minutes. Whisking constantly, gradually add melted butter in a steady stream; continue to whisk until fully combined and emulsified (no spots of fat should remain). Reserve saucepan.

Add dry ingredients into egg mixture in 3 additions, alternating with reserved sour cream mixture, beginning and ending with dry ingredients; whisk just until no lumps remain. Batter will be thin.

Scrape into pan and set on a rimmed baking sheet. Bake cake, rotating halfway through, until deep golden brown and a tester inserted into the center comes out clean, 60–80 minutes. Transfer pan to a wire rack and poke top of cake all over with a toothpick. Spoon 3 Tbsp. reserved reduced cider over cake; let cool 10 minutes.

Melt remaining 1 Tbsp. unsalted butter in reserved saucepan and mix into remaining 1 Tbsp. reduced cider.

Using parchment paper, lift cake onto rack and set rack inside rimmed baking sheet. Peel away parchment from sides. Brush warm butter-cider mixture over top and sides of cake. Sprinkle generously with cinnamon-sugar topping to coat every surface (use parchment to help rotate cake and collect any excess sugar). Remove parchment and let cool completely before slicing. Do Ahead: Cake can be made 4 days ahead. Store tightly wrapped at room temperature.