01 - Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, extending the sides for easy removal. In a small saucepan, simmer 1/2 cup apple cider over medium heat until reduced to 1/3 cup, approximately 8-10 minutes. Allow to cool slightly.
02 - Melt 1 cup butter in a saucepan over medium heat, swirling occasionally until it foams and develops a nutty brown color (about 3-4 minutes). Pour the browned butter into a large bowl and let it cool slightly. Whisk the packed light brown sugar into the browned butter until smooth. Incorporate the eggs, vanilla extract, and the reduced apple cider.
03 - In a separate mixing bowl, whisk together the all-purpose flour, baking powder, fine sea salt, ground cinnamon, and ground nutmeg. Gently fold the dry ingredients into the wet mixture until just combined. Carefully fold in the diced apple.
04 - Spread the batter evenly into the prepared baking pan. In a small bowl, combine the granulated sugar and ground cinnamon for the topping. Sprinkle this mixture evenly over the batter.
05 - Bake for 28-32 minutes, or until the edges appear set and a toothpick inserted into the center comes out mostly clean. Allow the blondies to cool completely in the pan. For the glaze, brown the remaining 1/4 cup butter in a saucepan. Remove from heat, then whisk in the sifted powdered sugar, a pinch of salt, and 2-3 tablespoons of apple cider until a smooth glaze forms.
06 - Drizzle the brown butter glaze over the cooled blondies. Allow the glaze to set before cutting the blondies into squares.