Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, cream butter and granulated sugar until light and fluffy.
Add eggs one at a time, mixing well after each. Stir in vanilla.
In a separate bowl, sift together flour, baking soda, baking powder, 1 teaspoon cinnamon, and salt. a
Alternate adding the dry ingredients and the buttermilk to the butter mixture. Sift in ⅓ of the dry ingredients, then add half the buttermilk. Repeat, starting and ending with flour. Mix gently after each addition.
Fold in the chopped apples. Spread the batter evenly into the prepared pan.
In a small bowl, mix 2 tablespoons brown sugar with 1 teaspoon cinnamon. Sprinkle evenly over the top.
Bake for 50-60 minutes, or until golden and a toothpick inserted in the center comes out clean.
Cool for 10–15 minutes before slicing.