Peel, core, and dice the Granny Smith apples into small pieces. Set aside until needed in the cake batter. Using tart apples like Granny Smith adds a nice contrast to the sweetness of the cake.

Preheat the oven to 350°F (175°C). In a large bowl, prepare the yellow cake mix according to the package directions for a 10-inch tube pan, typically by combining the cake mix with eggs, water, and oil as indicated on the box. Once the basic batter is ready, stir in the cinnamon powder and vanilla extract to evenly distribute the flavors. Fold the diced apples from Step 1 gently into the batter until they are just incorporated.

Pour the prepared batter into a greased 10-inch tube pan. Place the pan in the preheated oven and bake for about 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Once done, remove the cake from the oven and allow it to cool in the pan. I always let the cake cool completely before unmolding, as it helps prevent breaking and ensures a tender crumb.