Preheat oven to 350F (175C) and lightly grease the sides and bottom of a 9×13 baking pan.

Prepare topping by whisking together ½ cup sugar and 1 tablespoon cinnamon. Measure out just 2 tablespoons of the mixture and pour over your apples. Toss until all apple pieces are coated and then set aside.

In a separate large bowl, whisk together flour, sugars, baking powder, cinnamon, and salt until well-combined.

Make a well in the center and add oil, butter, eggs, buttermilk, and vanilla extract. Stir the liquid ingredients together until combined and then carefully fold the dry and wet ingredients together until well-combined (but don’t over-mix).

Add apples and stir until combined.

Spread batter into prepared pan and sprinkle the top evenly with remaining topping mixture. Transfer to 350F (175C) oven and bake for 40-45 minutes if using a metal pan or 50-55 minutes if using a glass dish. Cake will be done when a toothpick inserted in the center comes out clean or with a few moist crumbs (no wet batter!). Since it’s a somewhat dense cake, sometimes I like to use a knife to make a small cut in the center of the cake and make sure it’s baked through.