Preheat oven to 350°. In a 10-inch cast-iron skillet, melt butter over medium heat. Remove from heat. Sprinkle brown sugar over melted butter. Arrange citrus slices over brown sugar in skillet. Set aside.
In a large bowl, whisk together corn muffin mix, granulated sugar, eggs, buttermilk, and zest until well combined. Pour batter over citrus slices.
Bake until a wooden pick inserted in center comes out clean, about 35 minutes. Let cool in pan for
minutes. Carefully invert onto a flat serving plate.
Notes
*We used Jiffy brand corn muffin mix.
Recipe by Southern Cast Iron at https://southerncastiron.com/citrus-upside-cake/