Preheat oven to 400°. Melt 4 tbsp. of the butter in a 9" cast-iron skillet over medium-low heat. Add brown sugar and stir until well combined. Remove skillet from heat, add 1⁄4 cup of reserved pineapple juice, and stir well to combine. Arrange pineapple rings in a single layer in bottom of skillet and place a cherry in the center of each. Set skillet aside.
Put flour, granulated sugar, baking powder, and salt into a medium bowl and stir to combine. Melt remaining butter in a small pot over medium-low heat. Remove pot from heat, add milk and egg, and beat with a wooden spoon until well combined, about 1 minute. Pour milk mixture into flour mixture and beat until smooth, about 1 minute. Pour batter into skillet, covering the pineapple slices completely, and smooth out batter with a rubber spatula.
Bake cake until golden brown and a toothpick inserted in the middle comes out clean, 30–35 minutes. Remove from oven and let rest for 10 minutes. Cover skillet with a large plate and carefully invert the cake onto it. Serve warm or at room temperature, if you like.