This easy sheet cake is loaded with juicy apples and plenty of caramel flavor. The quick frosting has a little bit of tang thanks to sour cream -- though you can substitute Greek yogurt for a slightly lighter frosting with a similar flavor.
Preheat the oven to 350 degrees F. Spray a 9-by-13-inch baking pan with nonstick spray.
For the cake: Whisk the oil and melted butter together in a large bowl. Add the brown sugar and granulated sugar and mix well to combine. Add the eggs one at a time, whisking well after each addition. Add the vanilla extract and whisk to combine.
Whisk the flour, cinnamon, ginger, baking powder and salt together in a medium bowl. Add half of the flour mixture to the large bowl and stir with a rubber spatula to combine. Add the buttermilk and mix to combine. Add the remaining flour mixture and stir to combine. Fold in the apples.
Pour the batter into the prepared pan and spread into an even layer. Bake until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Let cool completely in the pan.
Meanwhile, make the frosting: In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and brown sugar together until very light and fluffy, 4 to 5 minutes. Add the confectioners' sugar in two additions, mixing well between additions and scraping the sides of the bowl down. Beat in the vanilla, cinnamon and sour cream.
Dollop the frosting over the top of the cooled cake and spread evenly. Serve immediately, or cover until ready to serve.