01 - Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a small saucepan, simmer 1 cup apple cider over medium heat until reduced to 1/3 cup (approximately 10-12 minutes). Allow to cool slightly and reserve 2-3 tablespoons for the glaze.
03 - In a medium bowl, combine flour, brown sugar, cinnamon, and nutmeg. Stir in melted butter until the mixture forms coarse crumbs. Set aside.
04 - In a separate medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg for the cake layer.
05 - In a large bowl, beat softened butter and brown sugar until creamy. Incorporate eggs one at a time, beating well after each addition. Stir in vanilla extract.
06 - Gradually add the dry ingredients to the wet mixture in two installments, alternating with the reduced apple cider. Mix until just combined, being careful not to overmix.
07 - Gently fold the diced apple into the cake batter.
08 - Spread the batter evenly into the prepared baking pan. Sprinkle the prepared streusel topping uniformly over the batter.
09 - Bake for 30-35 minutes, or until a toothpick inserted into the center emerges clean.
10 - Allow the bars to cool entirely in the baking pan.
11 - In a small bowl, whisk powdered sugar with the reserved 2-3 tablespoons of reduced apple cider until a smooth, pourable consistency is achieved.
12 - Drizzle the cider glaze over the cooled bars. Let the glaze set for 15 minutes before slicing into individual bars.