Preheat your oven to 350 degrees. Move an oven rack to the middle position. This is crucial for even baking.
Prepare your 9-inch by 13-inch cake pan by coating it with nonstick spray. This ensures your cake comes out easily after baking.
In a large bowl, whisk together all-purpose flour, brown sugar, granulated sugar, ground nutmeg, and salt. This mixture will create the dry ingredients that form the base of your cake.
Stir in vegetable oil until the mixture resembles coarse crumbs. This step is essential for achieving a crumbly texture that adds to the cake's charm.
Reserve ½ cup of the crumb mixture for topping later. It will create a lovely, crunchy layer on top of the cake.
In a separate medium bowl, whisk together buttermilk, egg, cinnamon, baking powder, and baking soda until smooth. This combination adds moisture and flavor to the cake.
Pour the wet ingredients into the bowl with the dry ingredients. Gently stir until just combined. Be careful not to over-mix; this will help keep the cake light and fluffy.
Pour the batter into the prepared cake pan. Don’t worry about getting it perfectly smooth; it will bake evenly.
Sprinkle the reserved crumb mixture over the top of the batter. This adds a delicious crunch and makes it visually appealing.
Bake in your preheated oven until the top is set and a toothpick inserted into the center comes out clean with a few crumbs attached. This should take about 40 minutes.
Once baked, remove the cake from the oven and let it cool in the pan for about 5 to 10 minutes before slicing. This waiting time allows the cake to set properly.