Pour 2 cups of apple cider into a small saucepan and set it over medium heat.

Let it come to a gentle boil, then reduce the heat to medium-low and let it simmer uncovered, stirring occasionally until reduced to roughly 1/2 cup.

Once reduced, pour it into a bowl or measuring cup and let it cool to room temperature.

Add the 1/2 cup of unsalted butter to the same saucepan and melt over medium heat, stirring as it melts.

Once it turns a medium amber color and smells nutty, pour it into a large mixing bowl and let it cool for 10 to 15 minutes.

Preheat your oven to 350 degrees Fahrenheit.

Grease a 9x5 inch loaf pan thoroughly with butter or cooking spray, then line it with parchment paper.

In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt.

Add both sugars to the bowl with the brown butter and whisk them together.

Add the eggs one at a time, whisking after each addition until fully incorporated.

Add the sour cream and vanilla extract, whisk again until smooth.

Finally, stir in the cooled apple cider reduction.

Pour the dry ingredients into the wet ingredients and fold gently until just combined.

Pour the batter into the prepared loaf pan and bake at 350 degrees Fahrenheit for 50 to 60 minutes.

Check doneness by inserting a toothpick into the center; it should come out with a few moist crumbs.

While the loaf cools for 10 minutes, melt the topping butter and mix together with cinnamon and sugar.

Brush the melted butter over the top of the loaf and sprinkle the cinnamon sugar mixture evenly.

Let the loaf cool on a wire rack for at least 20 minutes before slicing.