01 - Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, ensuring an overhang for easy removal of the finished bars.
02 - In a saucepan over medium-high heat, simmer the apple cider until it reduces to 1/4 cup. Allow it to cool.
03 - In a bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg.
04 - In a separate large bowl, whisk together the melted unsalted butter, light brown sugar, eggs, vanilla extract, and the cooled reduced apple cider until the mixture is smooth.
05 - Gradually add the dry ingredients to the wet mixture, stirring just until they are fully combined.
06 - Spread half of the batter evenly into the prepared baking pan. In a small bowl, mix the granulated sugar and cinnamon for the swirl; sprinkle half of this mixture over the batter. Add the remaining batter and gently spread it to cover. Top with the remaining cinnamon-sugar mixture.
07 - For the streusel topping, combine the all-purpose flour, light brown sugar, ground cinnamon, and salt in a bowl. Incorporate the cold, cubed unsalted butter by rubbing it in with your fingers or using a pastry cutter until the mixture resembles coarse crumbs. Distribute this evenly over the top layer.
08 - Bake for 30–35 minutes, or until a toothpick inserted into the center emerges clean and the top exhibits a golden-brown hue.
09 - Allow the bars to cool completely within the pan. To prepare the glaze, stir the powdered sugar with apple cider until a smooth consistency is achieved. Drizzle this over the cooled bars before cutting them into individual portions.