Preheat oven and prep pan:
Set oven to 325°F (165°C). Mist a 9x5-inch loaf pan with spray and line with parchment paper for easier removal after baking (optional).Reduce apple cider:
In a small saucepan, bring the apple cider to a boil. Let it boil roughly 10 minutes until it reduces to 1/2 cup (120 ml).Mix cider with sour cream:
Transfer the reduced cider to a small bowl; allow it to cool a bit. Whisk in the sour cream and vanilla extract.Melt butter:
In the same saucepan, melt the butter. Set aside to cool a little.Beat eggs and brown sugar:
Using a stand mixer or a whisk, beat the eggs and brown sugar together until the mixture is smooth and frothy.Stream in melted butter:
With the mixer on low speed (or while whisking by hand), slowly stream in the melted butter until fully incorporated.Whisk dry ingredients:
In another bowl, whisk the flour, cornstarch, baking powder, baking soda, salt, cinnamon, and nutmeg until uniform.Alternate dry and wet mixtures:
Add the dry blend to the egg mixture in three parts, alternating with the cider-sour cream mixture, starting and ending with dry ingredients. Stir just until combined; avoid overmixing.Bake loaf:
Pour the batter into the prepared pan. Bake for about 55 minutes, or until a toothpick inserted in the center emerges clean. Note: a slightly smaller pan may extend the baking time.Cool and remove from pan:
Let the pan rest on a wire rack for 10 minutes. Then lift the cake out using the parchment sling and let it cool for another 5 minutes.Coat with cinnamon sugar:
For the coating: combine granulated sugar and cinnamon. Brush the cake's exterior with melted butter, then generously coat with the cinnamon sugar. Apply to the top first, then each side. For sides, cup the sugar in your hand and press it against the cake; repeat until well covered.