Set a rack in the bottom-third of the oven and preheat to 350?/177?.

Grease an 8-inch by 8-inch square baking pan, then line with parchment paper.

Add 32 fluid ounces apple cider to a medium sauce pot and bring to a boil. Lower the heat and reduce for 25 ? 30 minutes until reduced to 2 cups, stirring occasionally.

Remove ? cup of the reduced cider and cool until lukewarm. Whisk in 60 grams sour cream and set aside.

Continue reducing the rest of the apple cider until it thickens to about ? cup, about 15 minutes. Season with a pinch of salt and set aside to cool.

Mix 62 grams all-purpose flour, 106 grams dark brown sugar, ? teaspoon ground cinnamon, and ? teaspoon kosher salt for the streusel. Add 57 grams melted butter and mix until small crumbs form. Chill in the fridge.

Mix 1 teaspoon flour and 1 tablespoon of the apple cider glaze in a small bowl. Toss to coat 5 ounces sliced apples.

In a stand mixer, cream 170 grams butter, 50 grams white sugar, and 53 grams brown sugar on medium speed until light and fluffy, about 4 ? 5 minutes.

Reduce speed to low, add 1 egg and 1 tablespoon vanilla extract. Mix on medium speed until whipped and fluffy.

On low speed, add ? teaspoon kosher salt, 1 ? teaspoons baking powder, ? teaspoon baking soda, and 1 ? teaspoons ground cinnamon.

Alternate adding 188 grams flour and the sour cream mixture until just combined. Fold in the sliced apples.

Transfer batter to the prepared pan and smooth in an even layer. Top with the streusel.

Bake for 30 ? 35 minutes, or until a toothpick comes out clean. Remove from the oven and drizzle 2 tablespoons of the glaze on top.

Cool for 10 minutes, then transfer to a wire rack. Cool completely before slicing.