Start by preheating your oven to 325°F and greasing a 9x13-inch baking dish.
In a large mixing bowl, whisk together the brown sugar, vegetable oil, egg, buttermilk, and vanilla extract until smooth.
Add the flour, baking soda, and salt, mixing just until combined. Be careful not to over mix the batter, as this helps keep the cake soft and tender.
Next, gently fold in the chopped apples. If you're using frozen apples, there's no need to thaw them first—simply stir them into the batter.
Pour the batter into the prepared baking pan.
In a separate small bowl, combine the granulated sugar, cinnamon, and melted butter until the mixture resembles damp sand. Sprinkle the cinnamon topping evenly over the cake batter.
Bake for 40–50 minutes, or until a toothpick inserted into the center comes out clean. Mine was perfect at around 45 minutes.
Allow the cake to cool slightly before slicing and serving.
