01 - Preheat oven to 350°F (175°C). Grease and line a 9x13-inch sheet pan with parchment paper.
02 - In a small saucepan, simmer apple cider over medium heat until it reduces to ½ cup. Allow it to cool to room temperature.
03 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt.
04 - In a large bowl, cream the softened butter with granulated sugar and light brown sugar until the mixture is light and fluffy. Beat in the eggs one at a time, then incorporate the vanilla extract.
05 - Mix in the sour cream and the reduced apple cider until the ingredients are well combined.
06 - Add the dry ingredients to the wet mixture and gently fold them together until just combined. Stir in the grated apple.
07 - Spread the batter evenly into the prepared sheet pan. Bake for 28–32 minutes, or until a toothpick inserted into the center emerges clean. Let the cake cool in the pan on a wire rack.
08 - For the sugar rubble, combine granulated sugar, ground cinnamon, melted butter, and a pinch of salt in a bowl. Mix until the mixture becomes crumbly and resembles wet sand.
09 - Generously sprinkle the sugar rubble over the cooled cake, pressing lightly to help it adhere. Cut into squares to serve.