Reduce the Apple Cider: Begin by pouring the apple cider into a small saucepan over medium-high heat, bringing it to a gentle boil or simmer until it reduces down to ¼ cup, which takes about 15-20 minutes.
Prepare Brown Butter: Melt unsalted butter in another saucepan over low heat until it foams, whisking frequently until it smells nutty and has toasted bits at the bottom; cool this brown butter in a heat-safe bowl for 15 minutes before chilling it further in the fridge.
Mix Dry Ingredients: In a separate bowl, combine all-purpose flour, baking powder, baking soda, cornstarch, spices, and salt, whisking thoroughly until well blended.
Combine Wet Ingredients: In a large mixing bowl, whisk together the cooled brown butter with both sugars until combined; then mix in the egg, yolk, reduced apple cider, and vanilla until smooth.
Form Cookie Dough: Gradually incorporate dry ingredients into the wet mixture until just combined; allow this dough to rest while preheating your oven to 375°F (190°C).
Bake Your Cookies: Using a medium cookie scoop, place balls of dough onto lined baking sheets spaced about three inches apart; bake them for 9-11 minutes or until edges are set and centers are slightly puffed before cooling on racks.