1 1/2 cups apple cider (reduced to 3/4 cup for best flavor)
2 large eggs
2 teaspoons vanilla extract
1/2 cup sour cream
1/2 cup unsalted butter, melted
1 cup light brown sugar, packed
2 tablespoons ground cinnamon
1/2 cup unsalted butter, softened (for swirl)
2 cups powdered sugar
2–3 tablespoons apple cider or milk (for glaze)
Instructions
Preheat your oven to 350°F (175°C). Generously grease a 9×13 inch baking pan with butter or non-stick cooking spray to ensure the cake releases easily after baking.
In a small saucepan over medium heat, simmer 1 1/2 cups of apple cider until it reduces to about 3/4 cup. This takes about 10-15 minutes and intensifies the apple flavor. Let it cool slightly before using.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt until well combined and free of lumps.
In a separate medium bowl, whisk together the reduced apple cider, eggs, vanilla extract, sour cream, and melted butter until the mixture is smooth and creamy.
Slowly pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix, as this can make the cake tough. Pour the batter into the prepared baking pan.
In a small bowl, cream together the softened butter, brown sugar, and ground cinnamon until it forms a paste-like consistency. Drop spoonfuls of this mixture over the top of the cake batter.
Using a butter knife or a skewer, gently swirl the cinnamon mixture into the batter to create a marble effect. Place in the oven and bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool for at least 15 minutes.
Whisk together the powdered sugar and apple cider (or milk) in a small bowl until smooth. Drizzle the glaze generously over the warm cake before serving.