Prepare the apples by peeling and dicing them into small pieces, about ½-inch cubes. Set aside.
In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer for 2-3 minutes until light and fluffy. Add the eggs one at a time, mixing well after each addition, and stir in the vanilla extract.
Gradually add the dry ingredients and buttermilk, alternating between them, starting and ending with the flour mixture. Mix until just combined, then gently fold in the diced apples.
In a separate bowl, combine the brown sugar, rolled oats, chopped nuts (if using), and ground nutmeg to create the crumble topping.
Pour half of the batter into a greased and floured baking pan, sprinkle half of the crumble topping over it, then pour the remaining batter on top and finish with the rest of the crumble topping.
Bake in a preheated oven for 40-45 minutes, checking for doneness with a toothpick. Allow to cool in the pan for 15 minutes before drizzling with cinnamon glaze.