Start by preheating your oven to 350°F (175°C). This step is crucial. If your oven isn’t hot enough, your cake won’t rise as it should.

Grease a 9x13-inch baking dish with a bit of vegetable oil or use non-stick cooking spray. This will ensure your cake releases easily once it's baked.

In a large mixing bowl, whisk together the flour, baking soda, salt, nutmeg, and half of the cinnamon sugar. This mixture will bring uniformity to your final batter.

In a separate bowl, combine the vegetable oil, buttermilk, vanilla extract, and egg. Whisk until fully blended. The oil helps keep the cake moist, and the buttermilk offers a tender crumb.

Gradually add the wet ingredients to the dry flour mixture. Stir gently to combine, making sure not to overmix. This is key to keeping it light and fluffy.

Once your batter is just combined, gently fold in the diced Honeycrisp apples. This step ensures that the apples are evenly distributed without breaking them down.

Pour the batter into your prepared baking dish. Spread it evenly with a spatula, ensuring a smooth top.

In a small bowl, mix the melted butter with the remaining granulated sugar and cinnamon. Drizzle this mixture over the top of your cake batter. It will create a delightful crunchy cinnamon sugar layer as it bakes.

Place your baking dish in the oven and bake for about 45-50 minutes. Keep an eye on it! You’ll know it’s done when a toothpick inserted into the center comes out clean.

Once baked, allow the cake to cool in the pan for a few minutes. Then transfer it to a wire rack to cool completely. Serve warm or at room temperature, perhaps with a scoop of ice cream.