Creating this Apple Cinnamon Sheet Cake is easy and rewarding if you follow these step-by-step instructions:

1. Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking sheet or pan.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, ground cinnamon, and salt. Set aside.
3. Cream Butter and Sugar: In a large bowl, beat the softened butter and granulated sugar until light and fluffy.
4. Add Eggs and Vanilla: Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
5. Combine Mixtures: Gradually add the dry mixture to the butter mixture, alternating with the buttermilk. Begin and end with the flour mixture, mixing until just combined.
6. Fold in Apple and Nuts: Gently fold in the diced apples and chopped nuts if using.
7. Pour Batter: Spread the batter evenly into the prepared baking pan.
8. Bake: Place in the preheated oven and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
9. Cool: Once baked, remove from the oven and allow to cool in the pan for about 15-20 minutes. Afterward, transfer the cake to a wire rack to cool completely.

These simple steps will have you on your way to baking a delicious Apple Cinnamon Sheet Cake in no time!