Preheat oven to 350°F. Spray a 9x13-inch baking pan with nonstick spray. Set aside.
In a large bowl, using a whisk, beat the cake mix, unsweetened applesauce, eggs, and vegetable oil together until well mixed, about 2 minutes.
15.25 ounces yellow cake mix, 1½ cups unsweetened applesauce, 3 large eggs, ¼ cup vegetable oil
Pour the cake batter into the prepared baking pan. Set aside.
In a separate small bowl, stir the brown sugar, ground cinnamon, and melted butter together until well mixed.
½ cup brown sugar, 1 tablespoon ground cinnamon, 3 tablespoons unsalted butter
Use a spoon to place small scoops of this brown sugar mixture evenly around the top of the cake.
Use a table knife to swirl this brown sugar mixture into the cake batter until it looks well combined.
Place cake in oven and bake for 26-27 minutes, or until a toothpick inserted in middle comes out clean. The top of the cake should look lightly brown.
Let cake cool completely before frosting.