Preheat your oven to 350°F. Grease a 9x13-inch baking pan (and line with parchment if you want easy removal).
Make the batter: Add all of the cake ingredients to a large mixing bowl. Mix with an electric mixer until smooth and fully combined.
Prep the apples: Peel, core, and dice the apples. In a separate bowl, toss the apples with sugar and cinnamon until evenly coated.
Fold in the apples: Add the apple mixture to the batter and gently mix just until the apples are evenly distributed.
Bake: Pour the batter into the prepared pan. Sprinkle the cinnamon topping mixture over the top, then use a knife to gently swirl it into the batter. Bake for 45–50 minutes, or until a toothpick inserted in the center comes out clean
Finish with icing: While the cake is still warm, poke holes all over the top using a skewer or fork. Pour the icing evenly over the cake so it soaks in. Let it set for a few minutes, then slice and serve.